define core protocol/port combinations (#2065)

* define core protocol/port combinations

* Update codegen

* Reword clause and include MUST requirement

* run ./hack/update-codegen.sh

apis/v1beta1/gateway_types.go

@@ -70,7 +70,13 @@ type GatewaySpec struct {
 	// At least one Listener MUST be specified.
 	//
 	// Each listener in a Gateway must have a unique combination of Hostname,
-	// Port, and Protocol.
+	// Port, and Protocol. Below combinations are considered Core and MUST be
+	// supported:
+	//
+	// 1. Port: 80, Protocol: HTTP
+	// 2. Port: 443, Protocol: HTTPS
+	//
+	// Port and protocol combinations not in this list are considered Extended.
 	//
 	// An implementation MAY group Listeners by Port and then collapse each
 	// group of Listeners into a single Listener if the implementation