chore(CI): lower the update frequency to every month as we don't have enough bandwidth

Author: Xmader

.github/workflows/update-mozcentral-version.yaml

@@ -2,7 +2,8 @@ name: 'Create pull requests to update mozilla-central version to the latest'
 
 on:
   schedule:
-    - cron: "00 14 * * 1" # run every Monday at 14:00 UTC (10:00 Eastern Daylight Time)
+    - cron: "00 14 */100,1-7 * 1" # run on the first Monday of each month at 14:00 UTC (10:00 Eastern Daylight Time)
+                                  # See https://blog.healthchecks.io/2022/09/schedule-cron-job-the-funky-way/
   workflow_call:
   workflow_dispatch: # or you can run it manually