Pass language version to DartFormatter().

Looks like I missed one constructor call when I fixed the others in:

#1307

pkgs/web_socket_conformance_tests/bin/generate_server_wrappers.dart

@@ -37,7 +37,8 @@ Future<StreamChannel<Object?>> startServer() async => spawnHybridUri(Uri(
 
 void main() async {
   final files = await Directory('lib/src').list().toList();
-  final formatter = DartFormatter();
+  final formatter = DartFormatter(
+      languageVersion: DartFormatter.latestLanguageVersion);
 
   files.where((file) => file.path.endsWith('_server.dart')).forEach((file) {
     final vmPath = file.path.replaceAll('_server.dart', '_server_vm.dart');