Update implementation_comparison.md

[Falco20019]

Could you please consider updating https://github.com/grpc/grpc-dotnet/blob/master/doc/implementation_comparison.md since it's over 2 years old and some stuff has happened. We did some checking ourselves but it would be great to have an up2date list now that everyone needs to change. Would help with knowing what fields might break support and need workarounds when migrating.

/CC @jtattermusch @JamesNK

[Falco20019]

Friendly ping since it's close to a month without reaction yet.

[JamesNK]

PR: #2240

Comment on the PR if there are changes you've seen that aren't mentioned.