Stale bot broken

Right now stale bot is not running correctly - it looks like it stopped working around a year ago (last pr I could find with stale bot activity). I'm guessing this was unintentionally and broken by probot not being supported anymore, just like the pr labeler (see https://issues.apache.org/jira/browse/BEAM-13779)

As a consequence of this breakage, we have a lot of stale prs that have been around for a while. As of the writing of this issue, we have 218 prs in the repo, with 129 older than 60 days old (59% stale assuming these don't have more recent activity). This makes the repo feel less active and makes it harder to see the legitimate PRs that have fallen through the cracks. We should fix this by adding the GitHub stale action