Sort labels by reference

What does this MR do?

Labels in a label note are sorted by reference (or reference_html), not by title. This may cause random CI failure if labels are sorted by title in different order than by reference.

EE version of this MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7284

What are the relevant issue numbers?

Closes #51241 (closed)

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