Fix miss-aligned approvers dropdown.

What does this MR do?

This MR fixes the issue where the approvers dropdown was miss-aligned on the Edit Merge request page.

What are the relevant issue numbers?

Closes #6500 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides