Add stricter SSH version check

Since this can only work for SSH version >6.5, this must be checked accordingly.

A few hints:

str=$(sshd --version 2>&1 | grep OpenSSH) ver=$(expr "$str" : 'OpenSSH_\([0-9]\{1,\}\.[0-9]\{1,\}\)') && echo "$ver"

sshd --version 2>&1 | sed '/OpenSSH_\([0-9]\{1,\}\.[0-9]\{1,\}\).*/!d;s//\1/;q'