WIP: Update issuable forms with newer dropdowns

What does this MR do?

Uses @iamphill's !4970 (merged) to swap out issuable dropdowns for newer dropdown_tag's, including updated specs

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

Why was this MR needed?

Screenshots (if relevant)

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 style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #12574 (closed)

Closes #21516 (closed)