fix: [container] Deprecate "EXPERIMENTAL" option for Gateway API (thi…

…s value has never been supported) (#5519)

* fix: Deprecate "EXPERIMENTAL" option for Gateway API (this value has never been supported)

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packages/google-container/protos/google/container/v1beta1/cluster_service.proto

@@ -5164,8 +5164,9 @@ message GatewayAPIConfig {
     // Gateway API support is disabled
     CHANNEL_DISABLED = 1;
 
+    // Deprecated: use CHANNEL_STANDARD instead.
     // Gateway API support is enabled, experimental CRDs are installed
-    CHANNEL_EXPERIMENTAL = 3;
+    CHANNEL_EXPERIMENTAL = 3 [deprecated = true];
 
     // Gateway API support is enabled, standard CRDs are installed
     CHANNEL_STANDARD = 4;