Reduce CI frequency.

We're having issues with github runners right now where they're queueing for long periods, and this has become disruptive.  We've gotten the flake rate under 2% total (<0.5% per build), and can ramp this up again later if we want to reduce it further.

PiperOrigin-RevId: 575227138

.github/workflows/test_runner.yml

@@ -12,9 +12,8 @@ name: Tests
 on:
   # continuous
   schedule:
-    # TODO Reduce this frequency
+    # Run every hour
-    # Run every 15 minutes for now to gather statistics
+    - cron: "0 * * * *"
-    - cron: "*/15 * * * *"
 
   # postsubmit
   push: