Bump SDK constraint to <4.0.0

Author: buijs-dev

pubspec.yaml

@@ -5,7 +5,7 @@ homepage: https://buijs.dev
 repository: https://github.com/buijs-dev/klutter-dart
 
 environment:
-  sdk: ">=2.15.1 <3.0.0"
+  sdk: ">=2.15.1 <4.0.0"
 
 dependencies:
   dart_style: ^2.2.3