Update pubspec.yaml

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: Zazo032

pubspec.yaml

@@ -4,7 +4,7 @@ version: 2.2.6
 homepage: https://www.agora.io
 repository: https://github.com/AgoraIO-Extensions/Agora-Flutter-RTM-SDK.git
 environment:
-  sdk: '>=3.9.0 <4.0.0'
+  sdk: '>=3.0.0 <4.0.0'
   flutter: '>=3.10.0'
 dependencies:
   flutter: