Remove pages sites asynchronously

What does this MR do?

This MR addresses #230694 (closed). For the sake of moving Puma to Kubernetes we want to restrict interactions between gitlab-rails and the Pages NFS drive to Sidekiq. This MR pushes pages removal into a new Sidekiq worker.

The change is feature flagged, we will track the rollout in #239318 (closed).

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done