Remove ujson (not before March 2026)

[brianschubert]

ujson 5.11.0 (released 2025-08-20) includes type hints.

https://github.com/ultrajson/ultrajson/releases/tag/5.11.0

[donBarbos]

I was planning to remove this step from CONTRIBUTING.md and generally revise the instructions for removing stubs after adding support "removal" PRs in stubsabot.
It seems that opening issue for stubs with the obsolete_since field is no longer necessary, since the stubsabot PR itself now serves as a reminder.