added sensitive flag to auth_string field

.changelog/4505.txt

@@ -0,0 +1,3 @@
+```release-note:enhancement
+redis : marked `auth_string` on the `resource_redis_instance` resource as sensitive
+```

google/resource_redis_instance.go

@@ -201,6 +201,7 @@ checked before each import/export operation.`,
 				Type:        schema.TypeString,
 				Description: "AUTH String set on the instance. This field will only be populated if auth_enabled is true.",
 				Computed:    true,
+				Sensitive:   true,
 			},
 			"project": {
 				Type:     schema.TypeString,

google/resource_redis_instance_test.go

@@ -143,8 +143,7 @@ resource "google_redis_instance" "test" {
     maxmemory-policy       = "noeviction"
     notify-keyspace-events = ""
   }
-  transit_encryption_mode = "SERVER_AUTHENTICATION"
-}
+  }
 `, name)
 }