Bump built_value_generator from 8.3.2 to 8.4.0

Bumps [built_value_generator](https://github.com/google/built_value.dart) from 8.3.2 to 8.4.0.
- [Release notes](https://github.com/google/built_value.dart/releases)
- [Changelog](https://github.com/google/built_value.dart/blob/master/CHANGELOG.md)
- [Commits](google/built_value.dart@v8.3.2...v8.4.0)

---
updated-dependencies:
- dependency-name: built_value_generator
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

pubspec.lock

@@ -98,7 +98,7 @@ packages:
       name: built_value_generator
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "8.3.2"
+    version: "8.4.0"
   characters:
     dependency: transitive
     description:

pubspec.yaml

@@ -42,7 +42,7 @@ dev_dependencies:
   flutter_test:
     sdk: flutter
   build_runner: ^2.2.0
-  built_value_generator: ^8.3.2
+  built_value_generator: ^8.4.0
   analyzer: ^4.3.1
   flutter_lints: ^2.0.1