Move from Microsoft.Bcl.Json.Sources to System.Text.Json (#9836)

- fix downgrade of CoreFx packages in core-sdk
- change eng/SharedFramework.Externsl.props
- remove System.Buffers and System.Runtime.CompilerServices.Unsafe references
  - System.Text.Json brings them in transitively
  - suppress baseline warning about System.Buffers
- remove more references from Microsoft.AspNetCore.SignalR.Protocols.Json project
  - Microsoft.AspNetCore.SignalR.Common brings necessary .NET Standard 2.0 packages in transitively
- regenerate ref project files

details:
- CoreFx no longer produces the Microsoft.Bcl.Json.Sources package
- System.Text.Json is now .NET Standard 2.0-compatible
- see also dotnet/corefx#37129 and aspnet/Extensions#1610