Introduce stale bot

[janhoy]

Following Lucene's lead and adding a stale bot that will label open, non-draft PRs as Stale after 60 days of inactivity. It is almost identical fork of Lucene's config: https://github.com/apache/lucene/blob/main/.github/workflows/stale.yml

I re-wrote the stale-message text a bit. I don't think random contributors can tag developers as reviewer, so I encouraged instead tagging people in a comment or emailing the dev-list.

[janhoy]

Being on the conservative side, I upped to 60 days before stale, and a daily budget of 10 PRs labeled.
This should create a minimum of friction and accomodate for the fact that in the Solr project, some development cycles are just slow without being stale. If noone touched a PR for 60 days, it's hard to argue that it is active, so a nudge by the bot may help drive some of those that just fell off the radar further.