upgrade flutter dependency

Author: lohanidamodar

templates/flutter/pubspec.yaml.twig

@@ -16,7 +16,7 @@ dependencies:
   http: ^0.13.4
   package_info_plus: 1.4.2
   path_provider: ^2.0.9
-  web_socket_channel: ^2.1.0
+  web_socket_channel: ^2.2.0
 
 dev_dependencies:
   flutter_lints: ^1.0.4