[connectivity_macos] fix flutter version constraint (#3604)

diff --git a/packages/connectivity/connectivity_macos/pubspec.yaml b/packages/connectivity/connectivity_macos/pubspec.yaml
index b8f36c8..860b164 100644
--- a/packages/connectivity/connectivity_macos/pubspec.yaml
+++ b/packages/connectivity/connectivity_macos/pubspec.yaml
@@ -11,7 +11,7 @@
 
 environment:
   sdk: ">=2.12.0-259.9.beta <3.0.0"
-  flutter: ">=1.10.0"
+  flutter: ">=1.20.0"
 
 dependencies:
   flutter: