Remove CAS code in snippets

[gewarren]

CAS is dead, even in .NET Framework (it's deprecated and not receiving any security or servicing fixes). Use the guidance in https://docs.microsoft.com/dotnet/fundamentals/syslib-diagnostics/syslib0003 to update our code snippets.

• dotnet/docs repo
• dotnet/dotnet-api-docs repo
• dotnet/samples (I didn't find anything obvious to update here)
• dotnet/docs-desktop