Enable even more frozen string in lib/gitlab

Enables frozens string for the following:

* lib/gitlab/conflict/**/*.rb
* lib/gitlab/cross_project_access/**/*.rb
* lib/gitlab/cycle_analytics/**/*.rb
* lib/gitlab/data_builder/**/*.rb
* lib/gitlab/database/**/*.rb
* lib/gitlab/dependency_linker/**/*.rb
* lib/gitlab/diff/**/*.rb
* lib/gitlab/downtime_check/**/*.rb
* lib/gitlab/email/**/*.rb
* lib/gitlab/etag_caching/**/*.rb

Partially addresses gitlab-org/gitlab-ce#47424.

changelogs/unreleased/frozen-string-lib-gitlab-more.yml

+---
+title: Enable even more frozen string in lib/gitlab/**/*.rb
+merge_request:
+author: gfyoung
+type: performance

lib/gitlab/conflict/file.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Conflict
     class File

lib/gitlab/conflict/file_collection.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Conflict
     class FileCollection

lib/gitlab/cross_project_access/check_collection.rb

+# frozen_string_literal: true
+
 module Gitlab
   class CrossProjectAccess
     class CheckCollection

lib/gitlab/cross_project_access/check_info.rb

+# frozen_string_literal: true
+
 module Gitlab
   class CrossProjectAccess
     class CheckInfo

lib/gitlab/cross_project_access/class_methods.rb

+# frozen_string_literal: true
+
 module Gitlab
   class CrossProjectAccess
     module ClassMethods

lib/gitlab/cycle_analytics/base_event_fetcher.rb

+# frozen_string_literal: true
+
 module Gitlab
   module CycleAnalytics
     class BaseEventFetcher

lib/gitlab/cycle_analytics/base_query.rb

+# frozen_string_literal: true
+
 module Gitlab
   module CycleAnalytics
     module BaseQuery

lib/gitlab/cycle_analytics/base_stage.rb

+# frozen_string_literal: true
+
 module Gitlab
   module CycleAnalytics
     class BaseStage

lib/gitlab/cycle_analytics/code_event_fetcher.rb

+# frozen_string_literal: true
+
 module Gitlab
   module CycleAnalytics
     class CodeEventFetcher < BaseEventFetcher

lib/gitlab/cycle_analytics/code_stage.rb

+# frozen_string_literal: true
+
 module Gitlab
   module CycleAnalytics
     class CodeStage < BaseStage

lib/gitlab/cycle_analytics/event_fetcher.rb

+# frozen_string_literal: true
+
 module Gitlab
   module CycleAnalytics
     module EventFetcher

lib/gitlab/cycle_analytics/issue_event_fetcher.rb

+# frozen_string_literal: true
+
 module Gitlab
   module CycleAnalytics
     class IssueEventFetcher < BaseEventFetcher

lib/gitlab/cycle_analytics/issue_stage.rb

+# frozen_string_literal: true
+
 module Gitlab
   module CycleAnalytics
     class IssueStage < BaseStage

lib/gitlab/cycle_analytics/metrics_tables.rb

+# frozen_string_literal: true
+
 module Gitlab
   module CycleAnalytics
     module MetricsTables

lib/gitlab/cycle_analytics/permissions.rb

+# frozen_string_literal: true
+
 module Gitlab
   module CycleAnalytics
     class Permissions

lib/gitlab/cycle_analytics/plan_event_fetcher.rb

+# frozen_string_literal: true
+
 module Gitlab
   module CycleAnalytics
     class PlanEventFetcher < BaseEventFetcher

lib/gitlab/cycle_analytics/plan_stage.rb

+# frozen_string_literal: true
+
 module Gitlab
   module CycleAnalytics
     class PlanStage < BaseStage

lib/gitlab/cycle_analytics/production_event_fetcher.rb

+# frozen_string_literal: true
+
 module Gitlab
   module CycleAnalytics
     class ProductionEventFetcher < IssueEventFetcher

lib/gitlab/cycle_analytics/production_helper.rb

+# frozen_string_literal: true
+
 module Gitlab
   module CycleAnalytics
     module ProductionHelper