bump iOS 11.0 to 12.4 in BUCK
bump iOS 11.0 to 12.4 in BUCK

Summary: Changelog: [iOS][Deprecated] Deprecating support for iOS/tvOS SDK 11.0, 12.4+ is now required

Reviewed By: fkgozali

Differential Revision: D34559477

fbshipit-source-id: cc0dc22a4b5158732ee77c224f351634b9a98aed

facebook/react-native,javache/react-native,facebook/react-native,janicduplessis/react-native,janicduplessis/react-native,facebook/react-native,janicduplessis/react-native,javache/react-native,facebook/react-native,facebook/react-native,facebook/react-native,facebook/react-native,javache/react-native,janicduplessis/react-native,janicduplessis/react-native,javache/react-native,javache/react-native,javache/react-native,javache/react-native,janicduplessis/react-native,facebook/react-native,javache/react-native,javache/react-native,facebook/react-native,janicduplessis/react-native,janicduplessis/react-native