Re-enable submit button after username change failure

What does this MR do?

Fixes issue where the "update username" button remained disabled after an AJAX request was complete.

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

I also fixed the .loading-username logic and removed references to .update-success and .update-failed blocks which I could not find any reference to anywhere else. I presume they were removed in an early revision and the frontend logic was just never updated to reflect this.

Why was this MR needed?

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

What are the relevant issue numbers?

Closes #25829 (closed)