feat: action for labeling and closing stale PRs (#17041)

Per discussion in the maintainer channel.

Uses a PR branch actions/stale#1145 of [actions/stale](https://github.com/actions/stale/) since it has a feature that will allow us to only consider PRs not sitting on the [#queue](https://bit.ly/3Ymuh0U). 

There's still placeholder text for the comments, but this is intended to be merged with the `debug-only` flag until we have the kinks sorted out, so we could fill in that text in a later PR.

.github/workflows/stale.yml

@@ -0,0 +1,25 @@
+name: 'Close stale issues and PRs'
+on:
+  schedule:
+    - cron: '30 1 * * *' # every day at 01:30 UTC
+  workflow_dispatch:
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    steps:
+      # until https://github.com/actions/stale/pull/1145 is merged
+      - uses: asterisk/github-actions-stale@main-only-matching-filter
+        with:
+          debug-only: 'true' # TODO: remove in follow-up PR after testing is done!
+          stale-pr-label: 'stale'
+          stale-pr-message: 'Message to comment on stale PRs.'
+          close-pr-label: 'closed-due-to-inactivity'
+          close-pr-message: 'Comment on the staled PRs while closed'
+          days-before-stale: 60
+          days-before-close: 120
+          # search string from the Zulip #queue link at https://bit.ly/4eo6brN
+          # "is:open is:pr -is:draft base:master sort:updated-asc status:success -label:blocked-by-other-PR -label:merge-conflict -label:awaiting-CI -label:WIP -label:awaiting-author -label:delegated -label:auto-merge-after-CI -label:ready-to-merge -label:please-adopt -label:help-wanted -label:awaiting-zulip"
+          # We want PRs _not_ on the queue, so we keep "is:pr -is:draft base:master" (is:open is added by the action by default) as a prefix for all queries and then negate the rest of the params in separate queries to simulate boolean OR (see https://github.com/actions/stale/pull/1145)
+          # except for label:auto-merge-after-CI and label:ready-to-merge which presumably will be noticed before they go stale
+          only-matching-filter: '[ "is:pr -is:draft base:master -status:success", "is:pr -is:draft base:master label:blocked-by-other-PR", "is:pr -is:draft base:master label:merge-conflict", "is:pr -is:draft base:master label:awaiting-CI", "is:pr -is:draft base:master label:WIP", "is:pr -is:draft base:master label:awaiting-author", "is:pr -is:draft base:master label:delegated", "is:pr -is:draft base:master label:please-adopt", "is:pr -is:draft base:master label:help-wanted", "is:pr -is:draft base:master label:awaiting-zulip" ]'