Add concurrency configuration to GitHub Actions

[vjik]

Q	A
Is bugfix?	❌
New feature?	✔️
Breaks BC?	❌

group: ${{ github.head_ref || github.run_id }} — in PR will be use head_ref (available in PR only), otherwise run_id (unique run ID, so never canceling).

cancel-in-progress: true — cancel any currently running workflow in the same concurrency group.

⚠ It would be good to test in a real package first.

[what-the-diff]

PR Summary

• Improvements to Workflow Concurrency
  This update includes additions to the concurrency settings of several GitHub workflow files including bc.yml, build.yml, composer-require-checker.yml, mutation.yml, rector.yml, and static.yml. This means our automated tasks should now run more efficiently, avoiding redundant operations and thus saving on time and system resources. Most importantly, it makes our system more reliable and reduces the likelihood of encountering errors due to workflow conflicts.