.github/stale.yml: enable the stale bot

Issues and PRs with no activity for 180 days will get marked as stale,
and if no further activity happens within 14 more days, the issue gets
closed.

This follows our established policy of not letting stalled bugs "get in
the way": https://curl.haxx.se/docs/bugs.html#Closing_off_stalled_bugs

Closes #1398