[ci] Specify limited concurrency for PR jobs (#31240)

There was a concurrency setting we hadn't enabled on jobs that are
primarily triggered for PRs. This meant that every update to the PR
would trigger new CI jobs without canceling any ones already in flight,
which can greatly slow down CI due to the number of jobs that need to
run.

This PR adds concurrency [based on the workflow name and PR number or
head
ref.](https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/control-the-concurrency-of-workflows-and-jobs)
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* __->__ #31240
* #31239