Draft: Prevent Project#destroy if associated builds exist

What does this MR do and why?

This MR adds a callback to prevent the destruction of Projects if Ci::Build records are still persisted for that Project. This shouldn't ever be the case, since we always destroy Projects via Projects::DestroyService and we now permanently use code to preemptively destroy builds without joining ci_ and non-ci_ tables.

This makes sure that no forbidden join queries are ever fired by dependent: :destroy, which we need to prevent for the database sharding to work well. If our optimized builds destruction code fails for any reason, we'll see errors on Project#delete_error about this.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

For demonstrative behavior:

1. Check out this branch locally
2. Change the association back to dependent: :destroy
3. Run the added tests and see the execution failures.
4. Change back to dependent: :restrict_with_exception and they'll pass again.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.