Fix and improve `Sortable.highest_label_priority`

What does this MR do?

Fix and improve Sortable.highest_label_priority

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

No.

Why was this MR needed?

Todos and Issuable's sorting by label priority is bugged currently.

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 #23928 (closed)