feat: Update required dart version (#658)

## What does this change?

dart_style v2.3.7 requries dart 3.0.0 or higher.

https://pub.dev/packages/dart_style/versions

Fixes #651 🎯

## Type of change

- [x] Breaking change (fix or feature that would cause existing
functionality to not work as expected)

Author: koji-1009

packages/command/pubspec.yaml

@@ -7,7 +7,7 @@ documentation: https://github.com/FlutterGen/flutter_gen
 issue_tracker: https://github.com/FlutterGen/flutter_gen/issues
 
 environment:
-  sdk: '>=2.17.0 <4.0.0'
+  sdk: ^3.0.0
 
 executables:
   fluttergen: flutter_gen_command

packages/core/pubspec.yaml

@@ -7,7 +7,7 @@ documentation: https://github.com/FlutterGen/flutter_gen
 issue_tracker: https://github.com/FlutterGen/flutter_gen/issues
 
 environment:
-  sdk: '>=2.17.0 <4.0.0'
+  sdk: ^3.0.0
 
 version_gen:
   path: lib/

packages/runner/pubspec.yaml

@@ -7,7 +7,7 @@ documentation: https://github.com/FlutterGen/flutter_gen
 issue_tracker: https://github.com/FlutterGen/flutter_gen/issues
 
 environment:
-  sdk: '>=2.17.0 <4.0.0'
+  sdk: ^3.0.0
 
 dependencies:
   flutter_gen_core: 5.9.0

pubspec.yaml

@@ -1,7 +1,7 @@
 name: flutter_gen_repository
 
 environment:
-  sdk: ">=2.17.0 <4.0.0"
+  sdk: ^3.0.0
 
 dev_dependencies:
   lints: any # Ignoring the version to allow editing across SDK versions.