Force legacy ws subprotocol when using gateway

Currently, ws subprotocols should be the same for notebook client and
gateway since there is no serializer/deserializer between the 2 streams
if subprotocols differ.

jupyter_server/gateway/connections.py

@@ -30,6 +30,10 @@ class GatewayWebSocketConnection(BaseKernelWebsocketConnection):
 
     retry = Int(0)
 
+    # When opening ws connection to gateway, server already negotiated subprotocol with notebook client.
+    # Same protocol must be used for client and gateway, so legacy ws subprotocol for client is enforced here.
+    kernel_ws_protocol = ""
+
     async def connect(self):
         """Connect to the socket."""
         # websocket is initialized before connection