Require the release version of Dart 3.0.0 (#925)

* Require the release version of Dart 3.0.0

* Update dart.yml

* Update pkgs/http/pubspec.yaml

Co-authored-by: Nate Bosch <[email protected]>

---------

Co-authored-by: Nate Bosch <[email protected]>

Author: brianquinlan

.github/workflows/dart.yml

@@ -4,7 +4,7 @@ description: A composable, multi-platform, Future-based API for HTTP requests.
 repository: https://github.com/dart-lang/http/tree/master/pkgs/http
 
 environment:
-  sdk: '>=3.0.0-417.4.beta <4.0.0'
+  sdk: ^3.0.0
 
 dependencies:
   async: ^2.5.0