RFC 6176 Prohibiting Secure Sockets Layer (SSL) Version 2.0, March 2011.
Unlike obsolete Sslv3 (and TLS1.0/1.0), Sslv2 should be forbidden.

Right now SslStream can support Sslv2 if underlying OS supports it.
At the moment, that seems to be only old versions of Windows.
While OpenSSL 1.0 could support it, all supported distributions are built without it. (Ubuntu16)

This maters as SslStream has craft and state handling to support v2 beyond SslProtocols.Ssl2 enum.
Perhaps we should strip it if passed in and throw PNSP if that is only protocol requested.