Improve Repository Sync (with new SystemHook)

What does this MR do?

• Adds an official repository_update event to the SystemHook
• Remove left-overs from old implementation
• Update existing SystemHooks associated with GeoNodes to switch to the new event.
• Use Gitlab::ExclusiveLease to prevent concurrent operations in the same repository
• New hook should be backward compatible with the Worker used with the temporary fix

Why was this MR needed?

See https://gitlab.com/gitlab-org/gitlab-ee/issues/1493

Documentation and backporting it to CE will be done in a separate MR.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #1463 (closed), #1493 (closed)

Related to gitlab-org/gitlab-ce#26325

CE Backport + Documentation: gitlab-org/gitlab-ce!11140

cc @stanhu @rspeicher @DouweM @dbalexandre