[meta] Requirements for tagging 12.0.0

Must-haves

• Address any regressions in previous milestones.
• Address any straggler deprecation cleanups

#3623168: Update deprecation notices in ajax.js

• Resolve critical 11.x -> 11.x upgrade path bugs.
• Resolve any 12.0.x-only security or data integrity issues.
• Resolve other 12-only upgrade blockers.

Should-haves