Drop support for Binary Serialization

[mmsmits]

Binary serialization is not recommended anymore (in fact, recommended against)

See: https://learn.microsoft.com/en-us/dotnet/standard/serialization/binaryformatter-security-guide

[ewoutkramer]

Checked with Martijn, Simplifier is not using binary serialization anymore.

[mmsmits]

We can't upgrade to .NET 9 if we don't do this.
Upgrade the tests project to .NET 9 while we are at it. There are a few other small things that we have to do for that.