Changes:
* Add support for a property, `IsAspNetCoreApp`, in the .csproj file of assemblies which are part of the shared framework.
* Remove unused dependencies
* Remove reference which have become part of 'netcoreapp3.0'
.NET Core 2.0 reached EOL last year. This removes multi-targeting our test projects and test assets to only use .NET Core 2.1 and .NET Framework 4.6.1.