Eliminate 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.

[CharliePoole]

I'm moving this to the 3.20 release, at least temporarily. If I'm able to get the work done in time, it will be released with 3.20 and then ported to V4.

Whether in V3 or V4, I plan to do this in two stages using separate PRs.

1. Simplify the structure of Messages

I currently maintain three versions of our BinarySerializationProtocol, all of which use BinaryForatter. There are two NUnit versions for V3 and V4 and a TestCentric version, which has evolved to use a more simple Message structure, represented by a single C# class. NUnit uses four classes. Moving to a single class with different message types recognized by the content of the message rather than it's .NET Type, is an improvement and will make elimination of BinaryFormatter a bit simpler.

2. Eliminate use of BinaryFormatter

Once the first stage is complete, it becomes possible to treat all messages simply as arrays of bytes and I believe we can eliminate any further encoding or decoding of the content.