Use latest extras

Ix.NET/Source/global.json

@@ -3,6 +3,6 @@
     "version": "3.0.100-preview4"
   },
   "msbuild-sdks": {
-    "MSBuild.Sdk.Extras": "2.0.0-preview.14"
+    "MSBuild.Sdk.Extras": "2.0.0-preview.21"
   }
 }