Bump flutter_lints in /packages/dropdown_button2_test

Bumps [flutter_lints](https://github.com/flutter/packages/tree/main/packages) from 2.0.3 to 4.0.0.
- [Release notes](https://github.com/flutter/packages/releases)
- [Commits](https://github.com/flutter/packages/commits/flutter_lints-v4.0.0/packages)

---
updated-dependencies:
- dependency-name: flutter_lints
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

packages/dropdown_button2_test/pubspec.yaml

@@ -14,7 +14,7 @@ dependencies:
     path: ../dropdown_button2
 
 dev_dependencies:
-  flutter_lints: ^2.0.1
+  flutter_lints: ^4.0.0
   flutter_test:
     sdk: flutter
   file: ^7.0.0