Changed 'Compare branches, tags or commit ranges' to 'Compare Git revisions'

What does this MR do?

I remember we had a quick discussion about what to call the expected input of the compare page. Today whilst doing !6766 (merged) I found these are called git revisions.

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

Why was this MR needed?

Screenshots (if relevant)

Dunno where that gradient came 😆 Screenshot issue not code.

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 it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?