* Replace probot/stale app with a Github Action
This creates a Github Actions workflow which seems to be the supported
way of automarking issues as stale. Adds a dry-run flag to test it out.
* small fixups
* cron typo
* disable unnecessary permissions
* use friendlier name
While I expect stale bot will close out 150 - 250 issues, that'll still leave us with 400+ open issues. My concern is that with a threshold of 6 months, most of these 400 issues will be in the same state 6 months from now and stale bot will annoy people by asking them if their issue is still valid too frequently.
Doubling the stale threshold to 1 year should mitigate this problem a bit I think.
