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.

Updates #5709

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

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: