Fix MR widget url

What does this MR do?

It fixes mr_widget_refresh_url in merge_request_helper where merge_request.source_branch was wrongly used instead of merge_request.target_branch

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?

fixes #27632 (closed)