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 (closed).

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

Tested by deploying the testing-gke-cluster-issue-11 branch of the Moodle deployment and checking that the maintenance window of the cluster is as expected in the Google Cloud console. Since we're in BST, this appears as being a window of 2-6AM:

Closes #11 (closed)