Adust Flutter constraint (#75773)

dev/devicelab/bin/tasks/plugin_dependencies_test.dart

@@ -102,7 +102,7 @@ dependencies:
 
 environment:
   sdk: ">=2.0.0-dev.28.0 <3.0.0"
-  flutter: ">=1.5.0 <2.0.0"
+  flutter: ">=1.5.0"
 ''', flush: true);
 
       section('Create plugin D without ios/ directory');