Add zone redundancy to private cluster API server load balancer

Updates the private cluster flavor to include availability zones
(1, 2, 3) on the API server internal load balancer for improved
high availability and resilience.

The private cluster E2E test is marked [OPTIONAL] so it will be
skipped in regions that don't support 3 availability zones.

Updates #5709

Author: bryan-cox

templates/cluster-template-private.yaml

@@ -7,6 +7,10 @@ spec:
     apiServerLB:
       name: ${CLUSTER_NAME}-internal-lb
       type: Internal
+      availabilityZones:
+        - "1"
+        - "2"
+        - "3"
     nodeOutboundLB:
       frontendIPsCount: 1
     controlPlaneOutboundLB: