Automatically close the latest milestone upon release

[dscho]

When a release creates a tag that points to the PR branch of a still-open "Rebase to "-class PR, we know that the branch should be pushed and the milestone should be closed out and a new "Next release" milestone should be opened.

Totally automatable.

[dscho]

#66 took care of pushing the main branch.

[dscho]

The main branch is now pushed, as of #66. This part is technically a bit different from closing the latest milestone and opening a new one because the main branch also needs to be pushed for -rc versions while the milestone needs to stay open for those.