[connectivity_macos] fix flutter version constraint (flutter#3604)

Author: Chris Yang

packages/connectivity/connectivity_macos/pubspec.yaml

@@ -11,7 +11,7 @@ flutter:
 
 environment:
   sdk: ">=2.12.0-259.9.beta <3.0.0"
-  flutter: ">=1.10.0"
+  flutter: ">=1.20.0"
 
 dependencies:
   flutter: