[d16-9] Update NuGet feed

bump maccore for dependencies updates

New commits in xamarin/maccore:

* xamarin/maccore@c50173e637 Remove dependency on XmlDocSync repo/tools (xamarin#2392)
* xamarin/maccore@9427bd123a Update NuGet.config to enforce a single feed (xamarin#2373)
* xamarin/maccore@146b36ed00 Keychain file computed incorrectly on Big Sur (xamarin#2388) (xamarin#2390)
* xamarin/maccore@4080d06db9 [mlaunch] Fix dependencies for Xcode 12.5 beta 1 (xamarin#2385)

Diff: https://github.com/xamarin/maccore/compare/428964bf32f06862eb4209129c6fb5b882c3b9be..c50173e637f7c10066f81dd8d130035f5672611a

NuGet.config

@@ -4,10 +4,14 @@
 		<add key="repositorypath" value="packages" />
 	</config>
 	<packageSources>
-		<add key="Nuget Official" value ="https://www.nuget.org/api/v2/" />
+		<clear />
+		<add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" protocolVersion="3" />
 		<add key="Dotnet arcade" value ="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" />
 		<add key="dotnet6" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet6/nuget/v3/index.json" />
 		<add key="dotnet5" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet5/nuget/v3/index.json" />
 	</packageSources>
+	<disabledPackageSources>
+		<clear />
+	</disabledPackageSources>
 </configuration>
 

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 := 428964bf32f06862eb4209129c6fb5b882c3b9be
+NEEDED_MACCORE_VERSION := c50173e637f7c10066f81dd8d130035f5672611a
 NEEDED_MACCORE_BRANCH := d16-9
 
 MACCORE_DIRECTORY := maccore