Resolve "Approvers don't appear when MR is created from a fork"

What does this MR do?

Use the target project, not the source project, for getting project members for the MR approvers dropdown.

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

🤷

Why was this MR needed?

We accidentally broke this! When creating an MR from a fork, @project is the fork, not upstream.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Added for this feature/bug
  • All builds are passing

What are the relevant issue numbers?

Closes #2556 (closed).