fix macos build

macos/Podfile

@@ -39,5 +39,8 @@ end
 post_install do |installer|
   installer.pods_project.targets.each do |target|
     flutter_additional_macos_build_settings(target)
+    target.build_configurations.each do |config|
+      config.build_settings['MACOSX_DEPLOYMENT_TARGET'] = '10.14.6'
+    end
   end
 end