Change CI concurrency policy to not queue on main (#8530)

Change CI concurrency policy to not queue on main (#8530)

Previously, we would only allow one job per branch/PR and only cancel old jobs on PRs.
This PR changes this policy to allow multiple jobs at once on main.
The way this is done basically follows https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#example-using-a-fallback-value.