[Breaking change] Removal of built-in WinRT support in .NET 5.0

With the work to provide sustainable and long term support for WinRT consumption in .NET, the built-in support has been removed. This is a high impact change to all existing libraries and application that rely on the built-in WinRT support.

The replacement for built-in WinRT support is the C#/WinRT tool chain.

For complete details see the official doc issue at dotnet/docs#18875.