fix the approvals_before_merge project fallback

What does this MR do?

This fix #3142 (closed) by making sure we use the correct target_project when evaluating the approvals count.

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

The current tests are done using a reflective MR (target_project == source_project), but I didn't find tests for MR that target a fork. The current change complies with the current test, but extensive tests could be added.

Why was this MR needed?

For a forked MR, the fallback project was always the project's fork, not the currently targeted project. This fixes this.

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?

Closes #3142 (closed)