Update dependency flutter_lints to v6

Generated by renovateBot

Author: renovate[bot]

sfu-ws/flutter/pubspec.yaml

@@ -16,7 +16,7 @@ dev_dependencies:
   flutter_test:
     sdk: flutter
 
-  flutter_lints: ^5.0.0
+  flutter_lints: ^6.0.0
 
 flutter:
   uses-material-design: true