openssl: use SSL_get_version to get connection protocol
Replace our bespoke get_ssl_version_txt in favor of SSL_get_version.
We can get rid of few lines of code, since SSL_get_version achieve
the exact same thing
Closes#6665
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Signed-off-by: Jean-Philippe Menil <jpmenil@gmail.com>