Make RepositoryUpdateMirrorWorker idempotent

In the event that multiple instances of these jobs are scheduled, it is
safe to batch them up into a single mirror update, which is by nature
idempotent.

Changelog: fixed

ee/app/workers/all_queues.yml

@@ -1127,7 +1127,7 @@
   :urgency: :low
   :resource_boundary: :unknown
   :weight: 1
-  :idempotent:
+  :idempotent: true
   :tags: []
 - :name: requirements_management_import_requirements_csv
   :worker_name: RequirementsManagement::ImportRequirementsCsvWorker

ee/app/workers/repository_update_mirror_worker.rb

 # frozen_string_literal: true
 
-class RepositoryUpdateMirrorWorker # rubocop:disable Scalability/IdempotentWorker
+class RepositoryUpdateMirrorWorker
   UpdateError = Class.new(StandardError)
 
   include ApplicationWorker
 
+  idempotent!
   data_consistency :always
   include Gitlab::ShellAdapter
   include ProjectStartImport