Move away from Circle CI towards Github Actions

[pedro-stanaka]

Summary

Can we move away from Circle CI and use only Github Actions? What was the history here and why we still use it?

Why?

Seems like Prometheus itself made the move 2y ago, but that is not a reason itself. I find depending on two CI systems quite clunky and we get much better integrated experience and faster CI if we use only one.

Notes

@matthiasr if you can provide context here, would be nice.

Relates to #326

[SuperQ]

There was some plan to move away, and it got partly there. But work on migrating stopped due to lack of volunteer time.

IMO we should probably finish the migration, but I dislike the promci setup. The action mono-repo is clunky to use. I would prefer to refactor this so it's easier to work with.