Rename priority sorting option to label priority

What does this MR do?

Rename priority sorting option to label priority

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

Why was this MR needed?

To clear up what exactly the priority sorting option sorts on.

Screenshots (if relevant)

old:

new:

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?

https://gitlab.com/gitlab-org/gitlab-ce/issues/29058#note_24861177

cc: @tauriedavis @smcgivern