Enable package validation

Re-enable NuGet package validation.

Author: martincostello

Directory.Build.props

@@ -65,8 +65,6 @@
 
   <PropertyGroup>
     <EnablePackageValidation>$(IsPackable)</EnablePackageValidation>
-    <!-- TODO Re-enable when the 9.0.0 packages are published to NuGet.org -->
-    <EnablePackageValidation>false</EnablePackageValidation>
   </PropertyGroup>
 
   <PropertyGroup>