not only add tag, but also make a proper release

Right now we use a daily cron job script to tag a specific commit on the release branch if that commit passed all dashboard tests. That causes those versions of the code to show up at https://github.com/fieldtrip/fieldtrip/tags with the version identifier YYYYMMDD.

Besides tagging, we could also make an explicit release using the `gh` command line client. See https://cli.github.com/manual/gh_release_create and https://docs.github.com/en/repositories/releasing-projects-on-github/about-releases