move cluster maintenance window

To avoid GMT/BST issues, move the maintenance window from 4-8AM UTC to
1-5AM UTC.

Google requires that the window be at least 4 hours and so this is
different from the 2-5AM specified by #11.

Take the opportunity to tidy up the recurrence specification and to
choose a rounder arbitrary date for the start time.

Closes #11

main.tf

@@ -75,12 +75,12 @@ resource "google_container_cluster" "cluster" {
     ]
   }
 
-  # Stop Google from upgrading cluster nodes during the work week
+  # Stop Google from upgrading cluster nodes during work hours.
   maintenance_policy {
     recurring_window {
-      recurrence = "FREQ=WEEKLY;BYDAY=MO,TU,WE,TH,FR,SA,SU"
-      end_time   = "2020-01-28T08:00:00Z"
-      start_time = "2020-01-28T04:00:00Z"
+      recurrence = "FREQ=DAILY"
+      end_time   = "2000-01-01T05:00:00Z"
+      start_time = "2000-01-01T01:00:00Z"
     }
   }
 }