IO::Socket::SSL's maintainer wants to migrate away from the value "sslv2/3"
for IO::Socket::SSL parameter SSL_version, which is where sslversion ends.
Adapt documentation and code to the value "sslv23", which is accepted in
IO::Socket::SSL since version 0.90 released in 2002.
For compatibility purposes still acept "sslv2/3", but convert it to "sslv23".