Add ActionsRML/delete-workflow-runs action

The amount of old workflows has grown to over 12k. Time for a cleanup.

Author: ivuorinen

.github/workflows/delete-old-workflows-monthly.yaml

@@ -0,0 +1,16 @@
+name: Delete old workflow runs
+on:
+  schedule:
+    - cron: '0 0 1 * *'
+# Run monthly, at 00:00 on the 1st day of month.
+
+jobs:
+  del_runs:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Delete workflow runs
+        uses: ActionsRML/delete-workflow-runs@main
+        with:
+          token: ${{ github.token }}
+          repository: ${{ github.repository }}
+          retain_days: 30