Disable terminating-gateway for property-override (#17605)

More validation is needed to ensure this behaves as expected; in the
meantime, align with docs and disable this proxy type.

agent/envoyextensions/builtin/property-override/property_override.go

@@ -106,8 +106,8 @@ var Ops = extensioncommon.StringSet{string(OpAdd): {}, string(OpRemove): {}}
 
 // validProxyTypes is the set of supported proxy types for this extension.
 var validProxyTypes = extensioncommon.StringSet{
+	// For now, we only support `connect-proxy`.
 	string(api.ServiceKindConnectProxy): struct{}{},
-	string(api.ServiceKindTerminatingGateway): struct{}{},
 }
 
 // Patch describes a single patch operation to modify the specific field of matching

agent/envoyextensions/builtin/property-override/property_override_test.go

@@ -618,7 +618,7 @@ func TestCanApply(t *testing.T) {
 		},
 		"invalid proxy type": {
 			ext: &propertyOverride{
-				ProxyType: api.ServiceKindTerminatingGateway,
+				ProxyType: api.ServiceKindConnectProxy,
 			},
 			conf: &extensioncommon.RuntimeConfig{
 				Kind: api.ServiceKindMeshGateway,