Ignore dns_config changes for Autopilot clusters (GoogleCloudPlatform…

…#8654)

Autopilot does not allow to modify dns_config.

Recently, in Autopilot the default dns_config has changed to be:
```
dns_config {
    cluster_dns        = "CLOUD_DNS"
    cluster_dns_domain = "cluster.local"
    cluster_dns_scope  = "CLUSTER_SCOPE"
  }
```

This breaks Autopilot customers as the terraform tries to converge dns_config back to null.

mmv1/third_party/terraform/services/container/resource_container_cluster.go.erb

@@ -1976,6 +1976,7 @@ func ResourceContainerCluster() *schema.Resource {
 				Optional:    true,
 				MaxItems:    1,
 				ForceNew:    true,
+				DiffSuppressFunc: suppressDiffForAutopilot,
 				Description: `Configuration for Cloud DNS for Kubernetes Engine.`,
 				Elem: &schema.Resource{
 					Schema: map[string]*schema.Schema{