Ignore dns_config changes for Autopilot clusters (#8654) (#15549)

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.

Signed-off-by: Modular Magician <[email protected]>

Author: modular-magician

.changelog/8654.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+container: updated `resource_container_cluster` to ignore `dns_config` diff when `enable_autopilot = true` 
+```

google/services/container/resource_container_cluster.go

@@ -1717,11 +1717,12 @@ func ResourceContainerCluster() *schema.Resource {
 				},
 			},
 			"dns_config": {
-				Type:        schema.TypeList,
-				Optional:    true,
-				MaxItems:    1,
-				ForceNew:    true,
-				Description: `Configuration for Cloud DNS for Kubernetes Engine.`,
+				Type:             schema.TypeList,
+				Optional:         true,
+				MaxItems:         1,
+				ForceNew:         true,
+				DiffSuppressFunc: suppressDiffForAutopilot,
+				Description:      `Configuration for Cloud DNS for Kubernetes Engine.`,
 				Elem: &schema.Resource{
 					Schema: map[string]*schema.Schema{
 						"cluster_dns": {