Replace old dropdowns on merge request compare page

What does this MR do and why?

Updates the dropdowns on the merge request compare to use GitLab UI listbox component

Screenshots or screen recordings

Screen_Recording_2023-01-20_at_12.02.44

How to set up and validate locally

• Enable mr_compare_dropdowns feature flag
• Visit new merge request page /-/merge_requests/new

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.

Closes #284687 (closed)