Add missing migration & update workflows

[robinheidrich]

This pull request was originally mainly to fix the errors in the workflows caused by the missing migration.

Then I noticed that the workflows were using outdated checkout and setup-python actions and looked through them all.

stale.yml: https://github.com/probot/stale was deprecated and probably no longer works. I replaced it with the stale action as recommended in the repository.

I hope the changes are ok.