Update dependency built_value_generator to v8

Author: renovate-bot

pubspec.yaml

@@ -31,7 +31,7 @@ dev_dependencies:
   flutter_test:
     sdk: flutter
   build_runner: ^2.0.0
-  built_value_generator: ^7.1.0
+  built_value_generator: ^8.0.0
 
 flutter:
   uses-material-design: true