System.Buffers ref to previously shipped version (#28428)
* Break netcoreapp20 build configuration from System.Buffers
* Build buffers with inbox frameworks using placeholders
* Update Compatibility pack System.Security.Cryptography.Cng to live prerelease dependency
* Update Compatibility Validation project and script to be able to restore for 2.1 and to include ASP.NET for ApiCatalog
* Merge conflicts and fix configurations to have ref harvested for all configurations
* Add netfx configuration to ref project for netfx vertical
* Remove not necessary build configurations
* PR Feedback