Stalebot for issues

[Gian Merlino]

There's been some discussion on GitHub about enabling stalebot (which we
use for PRs) for issues as well. Please check this PR out if you are
interested: https://github.com/apache/incubator-druid/pull/7936. It's a
follow up to https://github.com/apache/incubator-druid/pull/7927, which is
also recent.