Show information on upgrade libssh to 0.7.X
On Ubuntu below 16.04, libssh is still at version 0.6.X. This makes it impossible to connect to more securely configured SSH servers like shown in this bug issue: https://github.com/FreeRDP/Remmina/issues/983
This change helps people trying to figure out on how to connect to SSH servers using more secure algorithms which aren't supported by libssh 0.6.X.