Merge pull request #22 from koral--/renovate/flutter_lints-4.x

Update dependency flutter_lints to v4

Author: koral--

example/pubspec.yaml

@@ -20,7 +20,7 @@ dev_dependencies:
   flutter_test:
     sdk: flutter
 
-  flutter_lints: ^3.0.0
+  flutter_lints: ^4.0.0
   custom_lint: ^0.6.0
   plural_lint:
     path: ../