Integrate latest tooling

global.json

@@ -1,6 +1,6 @@
 {
   "msbuild-sdks": {
-    "WixToolset.Sdk": "4.0.0-build-0206"
+    "WixToolset.Sdk": "4.0.0-build-0211"
   },
   "sdk": {
     "allowPrerelease": false

src/wixext/UIExtensionFactory.cs

@@ -8,7 +8,7 @@ namespace WixToolset.UI
 
     public class UIExtensionFactory : BaseExtensionFactory
     {
-        protected override IEnumerable<Type> ExtensionTypes => new[]
+        protected override IReadOnlyCollection<Type> ExtensionTypes => new[]
         {
             typeof(UIExtensionData),
         };