Ensure the servicing attribute is set correctly on assemblies

[JunTaoLuo]

This is a followup item to the Arcade conversion originally filed at #13350.

The action item here is to verify whether the assembly is now correctly set during servicing. Details are at dotnet/arcade#1526

[dougbu]

Researched this issue a bit while working on dotnet/efcore#22649. That PR included the only change necessary to add the attribute to our EF and ASP.NET Core shipping assemblies.

Arcade defaults $(Serviceable) to true and none of dotnet/aspnetcore, dotnet/aspnetcore-tooling, or dotnet/efcore override that default. This covers the serviceable setting in generated *.nuspec files.

Further checks:

• Does dotnet/aspnetcore-tooling add [AssemblyMetadata("Serviceable", "true")] to all assemblies❔
  • No
• Do packages built using custom *.nuspec files include the correct setting❔
  • All but the VS.Redist.Common.AspNetCore.SharedFramework and VS.Redist.Common.AspNetCore.TargetingPack packages are fine

What remains is adding the missing [AssemblyMetadata(...)] in dotnet/aspnetcore-tooling and using $CommonMetadataElements) or something similar in dotnet/aspnetcore *.wixproj projects.

[dougbu]

The VS.Redist.Common.AspNetCore… packages don't ship and their .nuspec properties probably don't matter that much. I'll add <serviceable>true</serviceable> but doubt it will make much difference.