Add workflow for stale issues (#4161)

This adds a workflow to mark issues and PRs stale after the configured
period of time, followed by closing the issue/PR after a subsequent
period of time if there was no additional activity.

The `debug-only` option is so currently, so even when merged, it will do
a dry-run and not perform any actions. Once we inspect the initial logs
of the effect of an initial run (impacting existing issues), we can
adjust accordingly and then follow-up with making it active.

For the debug logs to be enabled, we do need to add a repository secret
named `ACTIONS_STEP_DEBUG` with a value set to `true` per [this
instruction](https://github.com/marketplace/actions/close-stale-issues#debugging).

.github/workflows/stale-issues.yaml

@@ -0,0 +1,21 @@
+name: stale-issues
+on:
+  schedule:
+    - cron: "30 1 * * *"
+
+permissions:
+  issues: write
+  pull-requests: write
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/stale@v8
+        with:
+          debug-only: true # Set until the behavior is tuned.
+          days-before-stale: 56 # Mark stale after 8 weeks (56 days) of inactivity
+          days-before-close: -1 # Disable auto-closing
+          exempt-all-milestones: true # Any issue/PR within a milestone will be omitted
+          #exempt-assigness: "foo,bar"  # Exempt issues/PRs assigned to particular users