chore: link validator once a month (#952)

Author: ennru

.github/workflows/link-validator.yml

@@ -4,7 +4,7 @@ on:
   workflow_dispatch:
   pull_request:
   schedule:
-    - cron:  '0 6 * * 1'
+    - cron:  '40 6 1 * *'
 
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   contents: read