feat: migrate to analyzer 9.0.0 API

- Replace Element2 API with Element API
- Update imports from element2.dart to element.dart
- Replace .firstFragment.name2 with .name
- Replace .firstFragment.metadata2.annotations with .metadata.annotations
- Replace .fields2 with .fields
- Replace .methods2 with .methods
- Remove duplicate imports in annotation_validator.dart and generated_class.dart
- Update test mocks to use Element instead of Element2

All generators now work correctly with analyzer 9.0.0:
- melos analyze: SUCCESS (0 issues)
- melos codegen: SUCCESS (13 outputs generated)
- melos bootstrap: SUCCESS (8 packages)

cherrypick_generator/pubspec.yaml

@@ -20,9 +20,9 @@ environment:
 # Add regular dependencies here.
 dependencies:
   cherrypick_annotations: ^4.0.0-dev.0
-  analyzer: ">=8.2.9 <10.0.1"
+  analyzer: ^9.0.0
   dart_style: ^3.0.0
-  build: ^3.0.0
+  build: ^4.0.4
   source_gen: ^4.2.0
   collection: ^1.18.0