Make the labels in the Compare form less confusing

What does this MR do?

• Improve the descriptive text
• Rename "from" to "Target" and "to" to "Source"
• Swap "Target" and "Source" to have the same order as in MRs
• Reworded "Switch base of comparison" to "Swap versions"

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

I didn't update the params name (i.e. from and to) as they're used in a lot of different context and they're not shown to the end-user in the compare page.

Why was this MR needed?

Because the page was confusing.

Screenshots (if relevant)

Before	After

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #21331 (closed)