chore: bump dart sdk constraints

Author: jpnurmi

pubspec.yaml

@@ -8,7 +8,7 @@ repository: https://github.com/jpnurmi/flutter_spinbox
 issue_tracker: https://github.com/jpnurmi/flutter_spinbox/issues
 
 environment:
-  sdk: ">=2.14.0 <3.0.0"
+  sdk: ">=2.14.0 <4.0.0"
   flutter: ">=3.7.0"
 
 dependencies: