fix: set CLOUD_DNS as provider for gke autopilot cluster

> Starting in August 2023, the default DNS provider for your new GKE Autopilot
> clusters using version 1.25.9-gke.400 or later and 1.26.4-gke.500 or later
> becomes Cloud DNS, at no extra charge. This change will be gradual and
> expected to be completed by Aug 12th.

Without this change, the default setting `PROVIDER_UNSPECIFIED` for
`dns_config.cluster_dns` is used with the `google_container_cluster`
ressource.

Thus running terraform apply to update parts of an deployment will
always recreate the cluster:

```
- dns_config { # forces replacement
  - cluster_dns        = "CLOUD_DNS" -> null
  - cluster_dns_domain = "cluster.local" -> null
  - cluster_dns_scope  = "CLUSTER_SCOPE" -> null
}
```

modules/beta-autopilot-private-cluster/cluster.tf

@@ -135,6 +135,13 @@ resource "google_container_cluster" "primary" {
     }
     workload_vulnerability_mode = var.workload_vulnerability_mode
   }
+
+  dns_config {
+    cluster_dns        = "CLOUD_DNS"
+    cluster_dns_domain = "cluster.local"
+    cluster_dns_scope  = "CLUSTER_SCOPE"
+  }
+
   ip_allocation_policy {
     cluster_secondary_range_name  = var.ip_range_pods
     services_secondary_range_name = var.ip_range_services

modules/beta-autopilot-public-cluster/cluster.tf

@@ -135,6 +135,13 @@ resource "google_container_cluster" "primary" {
     }
     workload_vulnerability_mode = var.workload_vulnerability_mode
   }
+
+  dns_config {
+    cluster_dns        = "CLOUD_DNS"
+    cluster_dns_domain = "cluster.local"
+    cluster_dns_scope  = "CLUSTER_SCOPE"
+  }
+
   ip_allocation_policy {
     cluster_secondary_range_name  = var.ip_range_pods
     services_secondary_range_name = var.ip_range_services