Add stale-issue-pruning to GitHub repo

Fixes #139

.github/workflows/prune-stale-issues-prs.yml

@@ -0,0 +1,15 @@
+name: "Close stale issues"
+on:
+  schedule:
+  - cron: "30 1 * * *"
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/stale@v3
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+        stale-issue-message: 'This issue has gone mouldy, because it has been open 30 days with no activity. Remove stale label or comment or this will be closed in 5 days'
+        days-before-stale: 30
+        days-before-close: 5