BinaryFormatter becomes obsolete -> Remove it with .NET 8.0

[vbreuss]

I tried executing the preview version of .NET 8.0 with System.IO.Abstractions and got the following test errors (see #1032)

The reason is, that the BinaryFormatter becomes obsolete.

I suggest to remove the support for Serialization under .NET8.0.

[fgreinacher]

Finally they did it 😅

Yes, let's exclude it for the .NET 8 build!

[github-actions]

This is addressed in release v19.2.87.