Fix ActiveRecord::Migration deprecations

Extending from ActiveRecord::Migration is deprecated, but was still used
in a bunch of places.

db/migrate/20181101191341_create_clusters_applications_cert_manager.rb

 # frozen_string_literal: true
-class CreateClustersApplicationsCertManager < ActiveRecord::Migration
+class CreateClustersApplicationsCertManager < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   DOWNTIME = false

db/migrate/20181108091549_cleanup_environments_external_url.rb

 # frozen_string_literal: true
-class CleanupEnvironmentsExternalUrl < ActiveRecord::Migration
+class CleanupEnvironmentsExternalUrl < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   DOWNTIME = false

db/migrate/20181115140140_add_encrypted_runners_token_to_settings.rb

 # frozen_string_literal: true
-class AddEncryptedRunnersTokenToSettings < ActiveRecord::Migration
+class AddEncryptedRunnersTokenToSettings < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   DOWNTIME = false

db/migrate/20181116050532_knative_external_ip.rb

@@ -3,7 +3,7 @@
 # See http://doc.gitlab.com/ce/development/migration_style_guide.html
 # for more information on how to write migrations for GitLab.
-class KnativeExternalIp < ActiveRecord::Migration
+class KnativeExternalIp < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   DOWNTIME = false

db/migrate/20181116141415_add_encrypted_runners_token_to_namespaces.rb

 # frozen_string_literal: true
-class AddEncryptedRunnersTokenToNamespaces < ActiveRecord::Migration
+class AddEncryptedRunnersTokenToNamespaces < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   DOWNTIME = false

db/migrate/20181116141504_add_encrypted_runners_token_to_projects.rb

 # frozen_string_literal: true
-class AddEncryptedRunnersTokenToProjects < ActiveRecord::Migration
+class AddEncryptedRunnersTokenToProjects < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   DOWNTIME = false

db/migrate/20181119081539_add_merge_request_id_to_ci_pipelines.rb

 # frozen_string_literal: true
-class AddMergeRequestIdToCiPipelines < ActiveRecord::Migration
+class AddMergeRequestIdToCiPipelines < ActiveRecord::Migration[4.2]
   DOWNTIME = false
   def up

db/migrate/20181120091639_add_foreign_key_to_ci_pipelines_merge_requests.rb

 # frozen_string_literal: true
-class AddForeignKeyToCiPipelinesMergeRequests < ActiveRecord::Migration
+class AddForeignKeyToCiPipelinesMergeRequests < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   DOWNTIME = false

db/migrate/20181120151656_add_token_encrypted_to_ci_runners.rb

 # frozen_string_literal: true
-class AddTokenEncryptedToCiRunners < ActiveRecord::Migration
+class AddTokenEncryptedToCiRunners < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   DOWNTIME = false

db/migrate/20181121101842_add_ci_builds_partial_index_on_project_id_and_status.rb

@@ -3,7 +3,7 @@
 # See http://doc.gitlab.com/ce/development/migration_style_guide.html
 # for more information on how to write migrations for GitLab.
-class AddCiBuildsPartialIndexOnProjectIdAndStatus < ActiveRecord::Migration
+class AddCiBuildsPartialIndexOnProjectIdAndStatus < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   DOWNTIME = false

db/migrate/20181121101843_remove_redundant_ci_builds_partial_index.rb

@@ -3,7 +3,7 @@
 # See http://doc.gitlab.com/ce/development/migration_style_guide.html
 # for more information on how to write migrations for GitLab.
-class RemoveRedundantCiBuildsPartialIndex < ActiveRecord::Migration
+class RemoveRedundantCiBuildsPartialIndex < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   DOWNTIME = false

db/post_migrate/20181010133639_backfill_store_project_full_path_in_repo.rb

 # frozen_string_literal: true
-class BackfillStoreProjectFullPathInRepo < ActiveRecord::Migration
+class BackfillStoreProjectFullPathInRepo < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   DOWNTIME       = false

db/post_migrate/20181026091631_migrate_forbidden_redirect_uris.rb

 # frozen_string_literal: true
-class MigrateForbiddenRedirectUris < ActiveRecord::Migration
+class MigrateForbiddenRedirectUris < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   DOWNTIME = false

db/post_migrate/20181121111200_schedule_runners_token_encryption.rb

 # frozen_string_literal: true
-class ScheduleRunnersTokenEncryption < ActiveRecord::Migration
+class ScheduleRunnersTokenEncryption < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   DOWNTIME = false

doc/development/background_migrations.md

@@ -211,7 +211,7 @@ existing data. Since we're dealing with a lot of rows we'll schedule jobs in
 batches instead of doing this one by one:
 ```ruby
-class ScheduleExtractServicesUrl < ActiveRecord::Migration
+class ScheduleExtractServicesUrl < ActiveRecord::Migration[4.2]
   disable_ddl_transaction!
   class Service < ActiveRecord::Base
@@ -242,7 +242,7 @@ jobs and manually run on any un-migrated rows. Such a migration would look like
 this:
 ```ruby
-class ConsumeRemainingExtractServicesUrlJobs < ActiveRecord::Migration
+class ConsumeRemainingExtractServicesUrlJobs < ActiveRecord::Migration[4.2]
   disable_ddl_transaction!
   class Service < ActiveRecord::Base

doc/development/migration_style_guide.md

@@ -67,7 +67,7 @@ body:
 For example:
 ```ruby
-class MyMigration < ActiveRecord::Migration
+class MyMigration < ActiveRecord::Migration[4.2]
   DOWNTIME = true
   DOWNTIME_REASON = 'This migration requires downtime because ...'
@@ -95,7 +95,7 @@ migration. For this to work your migration needs to include the module
 `Gitlab::Database::MultiThreadedMigration`:
 ```ruby
-class MyMigration < ActiveRecord::Migration
+class MyMigration < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   include Gitlab::Database::MultiThreadedMigration
 end
@@ -105,7 +105,7 @@ You can then use the method `with_multiple_threads` to perform work in separate
 threads. For example:
 ```ruby
-class MyMigration < ActiveRecord::Migration
+class MyMigration < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   include Gitlab::Database::MultiThreadedMigration
@@ -139,7 +139,7 @@ by calling the method `disable_ddl_transaction!` in the body of your migration
 class like so:
 ```ruby
-class MyMigration < ActiveRecord::Migration
+class MyMigration < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   disable_ddl_transaction!
@@ -167,7 +167,7 @@ the method `disable_ddl_transaction!` in the body of your migration class like
 so:
 ```ruby
-class MyMigration < ActiveRecord::Migration
+class MyMigration < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   disable_ddl_transaction!
@@ -193,7 +193,7 @@ Here's an example where we add a new column with a foreign key
 constraint. Note it includes `index: true` to create an index for it.
 ```ruby
-class Migration < ActiveRecord::Migration
+class Migration < ActiveRecord::Migration[4.2]
   def change
     add_reference :model, :other_model, index: true, foreign_key: { on_delete: :cascade }
@@ -216,7 +216,7 @@ For example, to add the column `foo` to the `projects` table with a default
 value of `10` you'd write the following:
 ```ruby
-class MyMigration < ActiveRecord::Migration
+class MyMigration < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   disable_ddl_transaction!
@@ -365,7 +365,7 @@ If you need more complex logic you can define and use models local to a
 migration. For example:
 ```ruby
-class MyMigration < ActiveRecord::Migration
+class MyMigration < ActiveRecord::Migration[4.2]
   class Project < ActiveRecord::Base
     self.table_name = 'projects'
   end

doc/development/prometheus_metrics.md

@@ -30,7 +30,7 @@ You might want to add additional database migration that makes a decision what t
 For example: you might be interested in migrating all dependent data to a different metric.
 ```ruby
-class ImportCommonMetrics < ActiveRecord::Migration
+class ImportCommonMetrics < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   require Rails.root.join('db/importers/common_metrics_importer.rb')

doc/development/sql.md

@@ -106,7 +106,7 @@ transaction. Transactions for migrations can be disabled using the following
 pattern:
 ```ruby
-class MigrationName < ActiveRecord::Migration
+class MigrationName < ActiveRecord::Migration[4.2]
   disable_ddl_transaction!
 end
 ```
@@ -114,7 +114,7 @@ end
 For example:
 ```ruby
-class AddUsersLowerUsernameEmailIndexes < ActiveRecord::Migration
+class AddUsersLowerUsernameEmailIndexes < ActiveRecord::Migration[4.2]
   disable_ddl_transaction!
   def up

doc/development/what_requires_downtime.md

@@ -88,7 +88,7 @@ renaming. For example
 ```ruby
 # A regular migration in db/migrate
-class RenameUsersUpdatedAtToUpdatedAtTimestamp < ActiveRecord::Migration
+class RenameUsersUpdatedAtToUpdatedAtTimestamp < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   disable_ddl_transaction!
@@ -118,7 +118,7 @@ We can perform this cleanup using
 ```ruby
 # A post-deployment migration in db/post_migrate
-class CleanupUsersUpdatedAtRename < ActiveRecord::Migration
+class CleanupUsersUpdatedAtRename < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   disable_ddl_transaction!
@@ -157,7 +157,7 @@ as follows:
 ```ruby
 # A regular migration in db/migrate
-class ChangeUsersUsernameStringToText < ActiveRecord::Migration
+class ChangeUsersUsernameStringToText < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   disable_ddl_transaction!
@@ -178,7 +178,7 @@ Next we need to clean up our changes using a post-deployment migration:
 ```ruby
 # A post-deployment migration in db/post_migrate
-class ChangeUsersUsernameStringToTextCleanup < ActiveRecord::Migration
+class ChangeUsersUsernameStringToTextCleanup < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   disable_ddl_transaction!
@@ -213,7 +213,7 @@ the work / load over a longer time period, without slowing down deployments.
 For example, to change the column type using a background migration:
 ```ruby
-class ExampleMigration < ActiveRecord::Migration
+class ExampleMigration < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   disable_ddl_transaction!
@@ -257,7 +257,7 @@ release) by a cleanup migration, which should steal from the queue and handle
 any remaining rows. For example:
 ```ruby
-class MigrateRemainingIssuesClosedAt < ActiveRecord::Migration
+class MigrateRemainingIssuesClosedAt < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
   DOWNTIME = false
@@ -322,7 +322,7 @@ Migrations can take advantage of this by using the method
 `add_concurrent_index`. For example:
 ```ruby
-class MyMigration < ActiveRecord::Migration
+class MyMigration < ActiveRecord::Migration[4.2]
   def up
     add_concurrent_index :projects, :column_name
   end

spec/rubocop/cop/migration/add_timestamps_spec.rb

@@ -11,7 +11,7 @@ describe RuboCop::Cop::Migration::AddTimestamps do
   subject(:cop) { described_class.new }
   let(:migration_with_add_timestamps) do
     %q(
-      class Users < ActiveRecord::Migration
+      class Users < ActiveRecord::Migration[4.2]
         DOWNTIME = false
         def change
@@ -24,7 +24,7 @@ describe RuboCop::Cop::Migration::AddTimestamps do
   let(:migration_without_add_timestamps) do
     %q(
-      class Users < ActiveRecord::Migration
+      class Users < ActiveRecord::Migration[4.2]
         DOWNTIME = false
         def change
@@ -36,7 +36,7 @@ describe RuboCop::Cop::Migration::AddTimestamps do
   let(:migration_with_add_timestamps_with_timezone) do
     %q(
-      class Users < ActiveRecord::Migration
+      class Users < ActiveRecord::Migration[4.2]
         DOWNTIME = false
         def change