build: upgrade Dart SDK constraint

- Updated SDK to ^3.8.0

Author: fulleni

pubspec.yaml

@@ -4,7 +4,7 @@ repository: https://github.com/headlines-toolkit/ht-api
 publish_to: none
 
 environment:
-  sdk: ">=3.0.0 <4.0.0"
+  sdk: ^3.8.0
 
 dependencies:
   dart_frog: ^1.1.0