chore: Linter updates: Void-Async

[nivisi]

So instead of having this:

void something() async {
  final variable = 0;
  ...
  await future();
}

We will have:

Future<void> something() {
  final variable = 0;
  ...
  return future();
}

It's good overall, but it also optimises the future's work in the new Dart 2.18 version.