Add several backend filters to the Vue version of the merge request list

What does this MR do and why?

This adds several backend filters for the upcoming Vue merge request list:

* :approver, filter by possible approver of a merge request

• :approved_by, filter by someone who approved a merge request
• :my_reaction_emoji, filter by emoji you've applied
• :merged_by, filter by merger of a merge request
• :release, filter by release tag

The frontend doesn't use these (yet), but for extra credit you could confirm by merging these MRs:

• Add My Reaction filter to Vue merge request lis... (!159807 - merged)
• Add merged-by filter to Vue merge request list app (!159808 - merged)
• Add approved-by filter to Vue merge request lis... (!160005 - merged)

Then enable the vue_merge_request_list feature flag and check the merge requests search bar of a project.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to:

• Add my-reaction filter to Vue merge request lis... (#442920 - closed)
• Add release filter to Vue merge request list app (#442919 - closed)
• Add approved-by filter to Vue merge request lis... (#442916 - closed)
• Add approver filter to Vue merge request list app (#442915 - closed)
• Add merged-by filter to Vue merge request list app (#442914 - closed)

app/graphql/resolvers/merge_requests_resolver.rb

@@ -145,7 +145,7 @@ def self.accept_reviewer
       argument :approved_by, [GraphQL::Types::String],
         required: false,
         as: :approved_by_usernames,
-        description: 'Usernames of the approvers.'
+        description: 'Usernames of approvers to exclude.'
       argument :assignee_usernames, [GraphQL::Types::String],
         as: :assignee_username,
         required: false,
@@ -156,16 +156,16 @@ def self.accept_reviewer
         description: 'Array of label names. All resolved merge requests will not have these labels.'
       argument :milestone_title, GraphQL::Types::String,
         required: false,
-        description: 'Title of the milestone.'
+        description: 'Title of the milestone to exclude.'
       argument :my_reaction_emoji, GraphQL::Types::String,
         required: false,
-        description: 'Filter by your reaction emoji.'
+        description: 'Filter by reaction emoji to exclude.'
       argument :release_tag, GraphQL::Types::String,
         required: false,
-        description: 'Filter by release tag.'
+        description: 'Filter by release tag to exclude.'
       argument :reviewer_username, GraphQL::Types::String,
         required: false,
-        description: 'Username of the reviewer.'
+        description: 'Username of the reviewer to exclude.'
     end
 
     validates mutually_exclusive: [:assignee_username, :assignee_wildcard_id]