Backport New SystemHook: `repository_update`

What does this MR do?

Backports new SystemHook from gitlab-org/gitlab-ee!1789

Are there points in the code the reviewer needs to double check?

Documentation

Why was this MR needed?

See gitlab-org/gitlab-ee!1789 for why we've created a new SystemHook

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 #26325 (closed)