feat(dart): rev the required Dart SDK version (#1848)

Author: devoncarew

internal/sidekick/internal/dart/templates/pubspec.yaml.mustache

@@ -28,7 +28,7 @@ publish_to: none
 {{/Codec.DoNotPublish}}
 
 environment:
-  sdk: ^3.6.0
+  sdk: ^3.9.0
 
 resolution: workspace
 {{#Codec.HasDependencies}}