feat: Upgrade to .NET 6

[karolberezicki]

Hi there @jason-roberts,

I've stumbled upon this library is some work codebase and I've spent a little bit of time to bump the library to .NET 6. Please let me know if you'd like me to change something 🙂

PS. I've changed the unit test to assert throws rather than fail, to better showcase, that this is the intended behavior and not something broke.