Fix: Switch to scheduled trigger for appropriate permission scope in workflow

Previously, the workflow was triggered by **opened pull requests** events.
However, testing revealed that this event, initiated by external GitHub users,
lacks the permission scope to comment on or close pull requests.

To address this, I've updated the workflow to trigger on a schedule. Scheduled
triggers aren't tied to external actors, thereby ensuring the necessary
permission scopes are available.

Pull Request: https://projects.blender.org/blender/blender/pulls/122363

.github/workflows/stale.yml

@@ -3,10 +3,10 @@
 
 name: Close GitHub Pull Requests
 
-# Trigger this Action when a pull request is opened.
+# Trigger this workflow every 12 hours.
 on:
-  pull_request:
-    types: [opened]
+  schedule:
+    - cron: '* */12 * * *'
 
 jobs:
   close_prs: