Virtual registries: cleanup steps for renaming VirtualRegistries::Packages::DestroyOrphanCachedResponsesWorker

Following https://docs.gitlab.com/ee/development/sidekiq/compatibility_across_updates.html#renaming-worker-classes

• Create new worker class and a switch with a feature flag. (%17.9) !177996 (merged)
  • Rollout issue: #513796 (closed)
• Rollout the feature flag. Enable the feature flag by default (%17.10) !182560 (merged)
• Remove feature flag and the old worker class (%17.11) !185394 (merged)