.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.
As a result of removing Razor's VS projects and upgrading all projects to netcoreapp3.0, we no longer need dependency variable for .NET Core 1.x and 2.x and Visual Studio packages.
Changes:
* Remove 'StandardTestTfm'
* Remove variables for .NET Core 1.0, 1.1, 2.0, 2.1, and 2.2
* Remove VSIX variables
* Stop generating 'branding.props' - this hasn't been used in a while and is no longer needed