upgrade min sdk version

examples/auth/flutter-native-google-auth/android/app/build.gradle

@@ -48,7 +48,7 @@ android {
         applicationId "com.example.myauthapp"
         // You can update the following values to match your application needs.
         // For more information, see: https://docs.flutter.dev/deployment/android#reviewing-the-gradle-build-configuration.
-        minSdkVersion flutter.minSdkVersion
+        minSdkVersion 20
         targetSdkVersion flutter.targetSdkVersion
         versionCode flutterVersionCode.toInteger()
         versionName flutterVersionName