Downgrade dependency on Dart SDK for notus to work with current Flutter beta

packages/notus/pubspec.yaml

 homepage: https://github.com/memspace/notus
 
 environment:
-  sdk: '>=2.0.0-dev.63.0 <2.0.0'
+  sdk: '>=2.0.0-dev.58.0 <2.0.0'
 
 dependencies:
   collection: ^1.14.6