Remove release channel UNSPECIFIED diff suppress (#4554) (#8595)

Signed-off-by: Modular Magician <magic-modules@google.com>

Author: modular-magician

.changelog/4554.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+container: fixed an issue where release channel UNSPECIFIED could not be set
+```

google/resource_container_cluster.go

@@ -906,10 +906,9 @@ func resourceContainerCluster() *schema.Resource {
 				Elem: &schema.Resource{
 					Schema: map[string]*schema.Schema{
 						"channel": {
-							Type:             schema.TypeString,
-							Required:         true,
-							ValidateFunc:     validation.StringInSlice([]string{"UNSPECIFIED", "RAPID", "REGULAR", "STABLE"}, false),
-							DiffSuppressFunc: emptyOrDefaultStringSuppress("UNSPECIFIED"),
+							Type:         schema.TypeString,
+							Required:     true,
+							ValidateFunc: validation.StringInSlice([]string{"UNSPECIFIED", "RAPID", "REGULAR", "STABLE"}, false),
 							Description: `The selected release channel. Accepted values are:
 * UNSPECIFIED: Not set.
 * RAPID: Weekly upgrade cadence; Early testers and developers who requires new features.
@@ -2961,12 +2960,12 @@ func flattenVerticalPodAutoscaling(c *containerBeta.VerticalPodAutoscaling) []ma
 
 func flattenReleaseChannel(c *containerBeta.ReleaseChannel) []map[string]interface{} {
 	result := []map[string]interface{}{}
-	if c != nil {
+	if c != nil && c.Channel != "" {
 		result = append(result, map[string]interface{}{
 			"channel": c.Channel,
 		})
 	} else {
-		// Explicitly set the release channel to the default.
+		// Explicitly set the release channel to the UNSPECIFIED.
 		result = append(result, map[string]interface{}{
 			"channel": "UNSPECIFIED",
 		})

google/resource_container_node_pool_test.go

@@ -1005,6 +1005,9 @@ resource "google_container_cluster" "cluster" {
   name               = "%s"
   location           = "us-central1-a"
   initial_node_count = 1
+  release_channel {
+	  channel = "UNSPECIFIED"
+  }
 }
 
 resource "google_container_node_pool" "np_with_management" {