Do we still need the merge_pr_* tags?

[gsnedders]

Historically we had to have the merge_pr_* tags to have somewhere to put the generated manifests.

This isn't actually true any more, now that we're using GitHub Actions for them, as we can simply pull them down as workflow artifacts.

It's unclear whether https://docs.github.com/en/rest/actions/artifacts?apiVersion=2022-11-28#list-artifacts-for-a-repository alone suffices, as that doesn't have a defined sort order, or we can simply do what we do now and start querying commits to see if we have a workflow run for them.

It would be nice to clear out the 32927 merge_pr_* tags we currently have, as this is pretty noisy when pulling.

See also: #117.