Increased min sdk version to 2.16.0

Author: Perondas

pubspec.yaml

@@ -3,7 +3,7 @@ version: 0.5.4
 description: A SIP UA stack for Flutter/Dart, based on flutter-webrtc, support iOS/Android/Destkop/Web.
 homepage: https://github.com/cloudwebrtc/dart-sip-ua
 environment:
-  sdk: ">=2.14.0 <3.0.0"
+  sdk: ">=2.16.0 <3.0.0"
   flutter: ">=2.0.0"
 
 dependencies: