Handle non-UTF-8 conflicts gracefully

What does this MR do?

If a conflict file isn't in a UTF-8-compatible encoding, we can't resolve it in the UI.

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

Is the regex the best way of doing this?

Why was this MR needed?

It's a bug in a new feature 😞

What are the relevant issue numbers?

Closes #21247 (closed).

Does this MR meet the acceptance criteria?

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