[ci] Run stale cache cleanup every 6 hours (#32739)

Run this a bit more frequently so we don't thrash `main` caches as
often.
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[//]: # (BEGIN SAPLING FOOTER)
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* __->__ #32739
* #32738

.github/workflows/shared_cleanup_stale_branch_caches.yml

@@ -3,7 +3,8 @@
 name: (Shared) Cleanup Stale Branch Caches
 on:
   schedule:
-    - cron: 0 0 * * *
+    # Every 6 hours
+    - cron: 0 */6 * * *
   workflow_dispatch:
 
 permissions: {}