RFR: 8282225: GHA: Allow one concurrent run per PR only

[Aleksey Shipilev]

Since last year, GHA allows concurrency control over GHA runs:
 https://github.blog/changelog/2021-04-19-github-actions-limit-workflow-run-or-job-concurrency/
 https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#concurrency

Our GHA workflows trigger on every PR update, sometimes doing multiple runs per PR. This is seldom useful and wastes resources with our very large jobs. For example, one can push a commit, quickly realize there is a mistake, push another commit, and this would do *two* GHA runs, both taking many hours.

I think we can say that only one run per branch is good, and all running/pending runs should be cancelled when a new run starts.

Additional testing:
 - [x] Verified queued run gets cancelled on new commit
 - [x] Verified in-progress run gets cancelled on new commit
 - [x] Verified in-progress run gets cancelled on merge
 - [x] Verified in-progress run gets cancelled on rebase + force-push

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Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/7570/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7570&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282225
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7570.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7570/head:pull/7570

PR: https://git.openjdk.java.net/jdk/pull/7570