[CI] Change update-sponsors workflow to run weekly every Tuesday (#852

)

I think it's better to have the workflow run every week than on fixed days of the month which leads to an irregular schedule. Tuesday is an arbitrary choice, but seems convenient.

.github/workflows/update-sponsors.yml

@@ -2,7 +2,7 @@ name: Update Sponsors
 
 on:
   schedule:
-    - cron:  '0 9 1,8,15,23 * *'
+    - cron:  '0 9 * * 2'
 
   workflow_dispatch: