ci: migrate to Github Actions Concurrency for cancelling workflows (#1158)
ci: migrate to Github Actions Concurrency for cancelling workflows (#1158)

Replaces `styfle/cancel-workflow-action` with [Github Actions concurrency](https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#concurrency). When the workflow has `cancel-in-progress: true`, this will cancel any currently running job or workflow in the same concurrency group which should be the same behaviour as `styfle/cancel-workflow-action`.

Closes #1151
googleapis/gapic-generator-python,googleapis/gapic-generator-python