manually set deployemtn target for pods

Author: denrase

flutter/example/ios/Podfile

@@ -43,5 +43,8 @@ end
 post_install do |installer|
   installer.pods_project.targets.each do |target|
     flutter_additional_ios_build_settings(target)
+    target.build_configurations.each do |config|
+      config.build_settings['IPHONEOS_DEPLOYMENT_TARGET'] = '11.0'
+    end
   end
 end