Enable more frozen string in app/services/**/*.rb

Partially addresses #47424.

app/services/lfs/file_transformer.rb

+# frozen_string_literal: true
+
 module Lfs
   # Usage: Calling `new_file` check to see if a file should be in LFS and
   #        return a transformed result with `content` and `encoding` to commit.

app/services/lfs/lock_file_service.rb

+# frozen_string_literal: true
+
 module Lfs
   class LockFileService < BaseService
     def execute

app/services/lfs/locks_finder_service.rb

+# frozen_string_literal: true
+
 module Lfs
   class LocksFinderService < BaseService
     def execute

app/services/lfs/unlock_file_service.rb

+# frozen_string_literal: true
+
 module Lfs
   class UnlockFileService < BaseService
     def execute

app/services/mattermost/create_team_service.rb

+# frozen_string_literal: true
+
 module Mattermost
   class CreateTeamService < ::BaseService
     def initialize(group, current_user)

app/services/members/approve_access_request_service.rb

+# frozen_string_literal: true
+
 module Members
   class ApproveAccessRequestService < Members::BaseService
     def execute(access_requester, skip_authorization: false, skip_log_audit_event: false)

app/services/members/base_service.rb

+# frozen_string_literal: true
+
 module Members
   class BaseService < ::BaseService
     # current_user - The user that performs the action

app/services/members/create_service.rb

+# frozen_string_literal: true
+
 module Members
   class CreateService < Members::BaseService
     DEFAULT_LIMIT = 100

app/services/members/destroy_service.rb

+# frozen_string_literal: true
+
 module Members
   class DestroyService < Members::BaseService
     def execute(member, skip_authorization: false)

app/services/members/request_access_service.rb

+# frozen_string_literal: true
+
 module Members
   class RequestAccessService < Members::BaseService
     def execute(source)

app/services/members/update_service.rb

+# frozen_string_literal: true
+
 module Members
   class UpdateService < Members::BaseService
     # returns the updated member

app/services/merge_requests/add_todo_when_build_fails_service.rb

+# frozen_string_literal: true
+
 module MergeRequests
   class AddTodoWhenBuildFailsService < MergeRequests::BaseService
     # Adds a todo to the parent merge_request when a CI build fails

app/services/merge_requests/assign_issues_service.rb

+# frozen_string_literal: true
+
 module MergeRequests
   class AssignIssuesService < BaseService
     def assignable_issues

app/services/merge_requests/base_service.rb

+# frozen_string_literal: true
+
 module MergeRequests
   class BaseService < ::IssuableBaseService
     def create_note(merge_request, state = merge_request.state)

app/services/merge_requests/build_service.rb

+# frozen_string_literal: true
+
 module MergeRequests
   class BuildService < MergeRequests::BaseService
     include Gitlab::Utils::StrongMemoize
@@ -140,7 +142,8 @@ module MergeRequests
       closes_issue = "Closes #{issue.to_reference}"
       if description.present?
-        merge_request.description += closes_issue.prepend("\n\n")
+        descr_parts = [merge_request.description, closes_issue]
+        merge_request.description = descr_parts.join("\n\n")
       else
         merge_request.description = closes_issue
       end
@@ -164,9 +167,11 @@ module MergeRequests
       return if merge_request.title.present?
       if issue_iid.present?
-        merge_request.title = "Resolve #{issue.to_reference}"
+        title_parts = ["Resolve #{issue.to_reference}"]
         branch_title = source_branch.downcase.remove(issue_iid.downcase).titleize.humanize
-        merge_request.title += " \"#{branch_title}\"" if branch_title.present?
+
+        title_parts << "\"#{branch_title}\"" if branch_title.present?
+        merge_request.title = title_parts.join(' ')
       end
     end

app/services/merge_requests/close_service.rb

+# frozen_string_literal: true
+
 module MergeRequests
   class CloseService < MergeRequests::BaseService
     def execute(merge_request, commit = nil)

app/services/merge_requests/conflicts/base_service.rb

+# frozen_string_literal: true
+
 module MergeRequests
   module Conflicts
     class BaseService

app/services/merge_requests/conflicts/list_service.rb

+# frozen_string_literal: true
+
 module MergeRequests
   module Conflicts
     class ListService < MergeRequests::Conflicts::BaseService

app/services/merge_requests/conflicts/resolve_service.rb

+# frozen_string_literal: true
+
 module MergeRequests
   module Conflicts
     class ResolveService < MergeRequests::Conflicts::BaseService

app/services/merge_requests/create_from_issue_service.rb

+# frozen_string_literal: true
+
 module MergeRequests
   class CreateFromIssueService < MergeRequests::CreateService
     def initialize(project, user, params)