Detect orphaned repositories

What does this MR do?

This is a follow up on the security issue : https://gitlab.com/gitlab-org/gitlab-ce/issues/36743

This was originally created here:

• https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2185
• https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2186

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

no

Why was this MR needed?

While the security issue is mitigated, people may want to cleanup leftovers with this script.

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
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?