Bump maccore. (#7142)

New commits in xamarin/maccore:

* xamarin/maccore@3e141c6779 [devops] Fix xcode11 provisioning (#2003)
* xamarin/maccore@86662c6050 Merge pull request #1999 from xamarin/swift-o-matic-doc-object-modeling
* xamarin/maccore@50822d35b3 Object Modeling documentation
* xamarin/maccore@f50226a11f Merge pull request #1998 from xamarin/swift-o-matic-doc-outline
* xamarin/maccore@d18f9ca2da functional outline documentation

Diff: 454248ba13..3e141c6779

mk/xamarin.mk

@@ -7,7 +7,7 @@ MONO_BRANCH    := $(shell cd $(MONO_PATH) 2> /dev/null && git symbolic-ref --sho
 endif
 
 ifdef ENABLE_XAMARIN
-NEEDED_MACCORE_VERSION := 454248ba1325b5395ee459fff05c66bb7211e835
+NEEDED_MACCORE_VERSION := 3e141c6779fe3d0d76f6960ed0e3edb729374f78
 NEEDED_MACCORE_BRANCH := master
 
 MACCORE_DIRECTORY := maccore