Add default alert to let us know when a cluster has been auto upgraded.

I do not think we currently have any alert/notification to let us know that a cluster has been auto-upgraded. I think we should at least get an email when this occurs so that we can check terraform still runs properly etc.

The following filter will get the upgrade log entry which will be useful.

resource.type="gke_cluster"
protoPayload.methodName="google.container.internal.ClusterManagerInternal.UpdateClusterInternal"