Time to get rid of BinaryFormatter?

[CharliePoole]

It's considered unsafe and is going away. See https://github.com/dotnet/designs/blob/main/accepted/2020/better-obsoletion/binaryformatter-obsoletion.md.

I've started work on this for my own engine (TestCentric/testcentric-engine#127) and I expect that the changes would apply almost directly to NUnit as well, possibly modified to allow for any changes made in that area since I left.