Changed compare dropdowns to dropdowns with search input

What does this MR do?

This changes the compare dropdowns from text inputs, that when clicked, open a dropdown of branches/tags to dropdowns that have a dropdown toggle and an isolated search input.

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

Why was this MR needed?

This was needed to fix the poor UX highlighted in #22221 (closed), where opening the dropdown showed an initially filtered set of results because the dropdown toggle was the dropdown filter itself. The compare page is always loaded with master as each branch/tag selection, so when opening the dropdown, it would only show results matching master.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #22221 (closed)