Run plone version check only once a week, or manually.

I look at this only rarely, so it is not needed to run this on every PR or commit.

.github/workflows/versioncheck.yml

@@ -1,8 +1,10 @@
 name: Plone Versioncheck
 
 on:
-  push:
-    branches: ["6.1"]
+  schedule:
+    - cron: "40 6 * * 0"
+  # Allow to run this workflow manually from the Actions tab
+  workflow_dispatch:
 
 jobs:
   build: