Resolve "Migrate issue labels and milestone to related merge request"

What does this MR do?

Labels and milestone were only migrated to a related MR when using the "Create merge request" button in the issue page. This MR adds the same behaviour when creating a MR that targets a specific issue (either by adding the issue number to the branch name or any other source)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

What are the relevant issue numbers?

Closes #50657 (closed)