[CI] Add maintenance window to premerge clusters

This patch adds an explicit maintenance window to the premerge clusters. This
is in response to some control plane upgrades that we recieved notice of that
will prevent access to the k8s control plane for ~15 minutes which means we
will not be able to start new jobs. We should have this anyways though as the
current node upgrade strategy also breaks jobs.

Author: boomanaiden154

premerge/gke_cluster/main.tf

@@ -19,6 +19,16 @@ resource "google_container_cluster" "llvm_premerge" {
   workload_identity_config {
     workload_pool = "llvm-premerge-checks.svc.id.goog"
   }
+
+  # We prefer that maintenance is done on weekends betwee 02:00 and 05:00
+  # UTC when commit traffic is low to avoid interruptions.
+  maintenance_policy {
+    recurring_window {
+      start_time = "2025-07-24T02:00:00Z"
+      end_time   = "2025-07-24T05:00:00Z"
+      recurrence = "FREQ=WEEKLY;BYDAY=SA,SU"
+    }
+  }
 }
 
 resource "google_container_node_pool" "llvm_premerge_linux_service" {