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?
Does this MR meet the acceptance criteria?
- Changelog entry added, if necessary
API support added
- Added for this feature/bug
- All builds are passing
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?