Fix incorrect instances of `GraphQL::ID_TYPE`

Since the `GraphQL::ID_TYPE` usages should represent globally unique
ids, this changes some fields for which this is not the case into
strings.

The `ID_TYPE` is a specialised, so this change should be backwards
compatible.

https://graphql-ruby.org/type_definitions/scalars.html

app/graphql/mutations/merge_requests/base.rb

@@ -10,7 +10,7 @@ module Mutations
                required: true,
                description: "The project the merge request to mutate is in"
-      argument :iid, GraphQL::ID_TYPE,
+      argument :iid, GraphQL::STRING_TYPE,
                required: true,
                description: "The iid of the merge request to mutate"

app/graphql/resolvers/issues_resolver.rb

@@ -2,11 +2,11 @@
 module Resolvers
   class IssuesResolver < BaseResolver
-    argument :iid, GraphQL::ID_TYPE,
+    argument :iid, GraphQL::STRING_TYPE,
               required: false,
               description: 'The IID of the issue, e.g., "1"'
-    argument :iids, [GraphQL::ID_TYPE],
+    argument :iids, [GraphQL::STRING_TYPE],
               required: false,
               description: 'The list of IIDs of issues, e.g., [1, 2]'
     argument :state, Types::IssuableStateEnum,

app/graphql/resolvers/merge_requests_resolver.rb

@@ -2,11 +2,11 @@
 module Resolvers
   class MergeRequestsResolver < BaseResolver
-    argument :iid, GraphQL::ID_TYPE,
+    argument :iid, GraphQL::STRING_TYPE,
               required: false,
               description: 'The IID of the merge request, e.g., "1"'
-    argument :iids, [GraphQL::ID_TYPE],
+    argument :iids, [GraphQL::STRING_TYPE],
               required: false,
               description: 'The list of IIDs of issues, e.g., [1, 2]'

app/graphql/types/ci/pipeline_type.rb

@@ -10,7 +10,7 @@ module Types
       expose_permissions Types::PermissionTypes::Ci::Pipeline
       field :id, GraphQL::ID_TYPE, null: false
-      field :iid, GraphQL::ID_TYPE, null: false
+      field :iid, GraphQL::STRING_TYPE, null: false
       field :sha, GraphQL::STRING_TYPE, null: false
       field :before_sha, GraphQL::STRING_TYPE, null: true

app/graphql/types/merge_request_type.rb

@@ -11,7 +11,7 @@ module Types
     present_using MergeRequestPresenter
     field :id, GraphQL::ID_TYPE, null: false
-    field :iid, GraphQL::ID_TYPE, null: false
+    field :iid, GraphQL::STRING_TYPE, null: false
     field :title, GraphQL::STRING_TYPE, null: false
     field :description, GraphQL::STRING_TYPE, null: true
     field :state, MergeRequestStateEnum, null: false

spec/requests/api/graphql/mutations/merge_requests/set_wip_spec.rb

@@ -11,7 +11,7 @@ describe 'Setting WIP status of a merge request' do
   let(:mutation) do
     variables = {
       project_path: project.full_path,
-      iid: merge_request.iid
+      iid: merge_request.iid.to_s
     }
     graphql_mutation(:merge_request_set_wip, variables.merge(input))
   end