Refactor compare version dropdowns

What does this MR do?

Context: See this closed MR for discussion and previous implementation beginnings !26918 (closed)

This is a refactor of the commit versions dropdowns. When developing displaying the head version for #27008 (closed) it became apparent that the complexity of the code for these dropdowns was too much. We needed to cut down the tech debt in order to confidently ship the feature.

Relates to: #198457 (closed)

Relates to: #209071 (closed)

Screenshots

n/a

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content