Fix deletion of deploy keys linked to other projects

What does this MR do?

When a project is deleted, this MR ensures that we don't delete its deploy keys if they are linked to other projects.

Why was this MR needed?

https://gitlab.com/gitlab-org/gitlab-ce/issues/35630

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
• 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
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Fixes #35630