Switch the release trigger from PR to a git tag. In practice this would
mean tagging a commit in master branch and pushing it with
`git push --tags`.
The benefit of this is that tagging is already an event that is reserved
for maintainers, so we can remove the need for verifying whether the
event was done by a maintainer.
We also no longer need to keep track of the tag. Previously the trigger
was a PR which has a different ref from the tag, so manual bookkeeping
was required to ensure github used the tag reference instead of the PR
reference. Having the tagging itself be the trigger removes this need
entirely as the default checkout will already use the tag as reference.
