ci: Use GH concurrency queue instead of turnstile in pr-is-up-to-date.yml

[anomiex]

Proposed changes

GitHub has recently added the ability to have an actual queue (max 100 entries) for its concurrency feature. This should perform better than turnstile, so let's try it out.

Also, there's a bit of cruft in the file: when I originally wrote it, I thought of it as one feature so I put it all in one file, and then had to get a bit hacky to avoid always having a skipped job in the PR check list. But we can also avoid that, more cleanly, by just having two independent workflows.

Related product discussion/links

p1779371671037339-slack-C05Q5HSS013

Does this pull request change what data or activity we track or use?

No

Testing instructions

• Fork, set up secrets in the fork, merge this, and then try stuff and see how it runs.
  • https://github.com/anomiex/jetpack/actions/workflows/pr-is-up-to-date-trunk-push.yml
  • https://github.com/anomiex/jetpack/actions/workflows/pr-is-up-to-date.yml

[github-actions]

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