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

Author: philIip

tools/build_defs/oss/rn_defs.bzl

@@ -69,7 +69,7 @@ JNI_TARGET = "//ReactAndroid/src/main/jni/first-party/jni-hack:jni-hack"
 KEYSTORE_TARGET = "//keystores:debug"
 
 # Minimum supported iOS version for RN
-REACT_NATIVE_TARGET_IOS_SDK = "11.0"
+REACT_NATIVE_TARGET_IOS_SDK = "12.4"
 
 def get_apple_inspector_flags():
     return []