Bump maccore to treat Xcode GM as beta (needed for bots) (#6985)

New commits in xamarin/maccore:

* xamarin/maccore@a9847b2ea5 [tests] Treat Xcode GM as beta so we don't have to update all devices to GM (#1964)

Diff: 079bd45a0f..a9847b2ea5

mk/xamarin.mk

@@ -1,5 +1,5 @@
 ifdef ENABLE_XAMARIN
-NEEDED_MACCORE_VERSION := 079bd45a0fdb7f054f40d6ba883ab6f4af8772d6
+NEEDED_MACCORE_VERSION := a9847b2ea52c77d46fc19f7444b07b93d2f80f4c
 NEEDED_MACCORE_BRANCH := xcode11
 
 MACCORE_DIRECTORY := maccore