[ci] Opt in to auto .NET 6 VS PR creation (#12414)

Context: https://github.com/xamarin/yaml-templates/pull/124

The VS Insertion stage will now automatically open a PR against VS main
once approved.  The primary limitation with the current workflow is that
there is no good way to automatically create a PR against a branch other
than VS main.  This can be addressed at a later date, as we can still do
manual insertions into release branches during QB mode as needed

dotnet/Workloads/vs-workload.template.props

@@ -2,6 +2,7 @@
 <Project>
   <PropertyGroup>
     <TargetName>Microsoft.NET.Sdk.MaciOS.Workload</TargetName>
+    <ManifestBuildVersion>@IOS_WORKLOAD_VERSION@</ManifestBuildVersion>
   </PropertyGroup>
   <ItemGroup>
     <!-- Shorten package names to avoid long path caching issues in Visual Studio -->