Update the reference assemblies we ship in mono.

diff --git a/external/binary-reference-assemblies b/external/binary-reference-assemblies
index 4abf9a74bd9a807380ba8f55a73b322625e15565..7120b21603354fced392a43ab7c581d0114fecac 160000 (submodule)
--- a/external/binary-reference-assemblies
+++ b/external/binary-reference-assemblies
@@ -1 +1 @@
-Subproject commit 4abf9a74bd9a807380ba8f55a73b322625e15565
+Subproject commit 7120b21603354fced392a43ab7c581d0114fecac