Using GHA's `concurrency` syntax

Currently we have this logic to prevent multiple runs on `main` for different commits:

https://github.com/conda-forge/admin-requests/blob/cd3686599e10a654b7f4ec2a2cce46ab78db074b/.github/workflows/main.yml#L14-L20

Another option would be to use [GHA's `concurrency` syntax]( https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#concurrency ). Here's [an example]( https://github.com/dask/dask/blob/23a75632b7644f1289a760031d9ba85a35cbfd20/.github/workflows/conda.yml#L12-L16 ).

If this seems reasonable, GHA configs in other repos may be able to employ the same strategy.