Refactor jQuery dropdown implementation in "new merge request" form to GitLab UI GlDropdown

The "new merge request" form currently utilizes our old deprecated jQuery-based dropdown implementation and should be replaced with a new implementation based upon GitLab UI's GlDropdown component. The implementation is defined here:

• app/assets/javascripts/compare_autocomplete.js (soon to me moved to ~/pages/projects/merge_requests/creations/new in !61416 (merged))

This code was originally used in both the "New Merge Request" form and the "Compare Revisions" form, though it appears the later has been refactored and merely requires some vestigial code to be removed (see !61416 (merged))

New Merge Request	Compare Revisions

Since the "Repository -> Compare" page already contains a similar form written in Vue, I believe we could find a way to re-use the code in both places. Once we've replaced this implementation, we can remove compare_autocomplete.js as part of &4284