Update package Category (#15058)

See dotnet/source-build#5070

Also remove unnecessary PublishingVersion property.

Author: ViktorHofer

eng/Publishing.props

@@ -1,6 +1,9 @@
-<?xml version="1.0" encoding="utf-8"?>
 <Project>
-   <PropertyGroup>
-      <PublishingVersion>3</PublishingVersion>
-   </PropertyGroup>
+
+  <!-- Update Artifacts with Kind=Package to have additional metadata item Category="ToolingPackage".
+       Depending on channel configuration, this means that these assets could be pushed to a different feed. -->
+  <ItemGroup>
+    <Artifact Update="@(Artifact->WithMetadataValue('Kind', 'Package'))" Category="ToolingPackage" />
+  </ItemGroup>
+
 </Project>