fix incorrect name date log sessions ssh

The option to log ssh sessions with a filename filled automatically with a local datetime in ISO 8601 format is broken.

This commit aims to fix it by following the format specifiers defined here : https://developer.gnome.org/glib/stable/glib-GDateTime.html#g-date-time-format

I think it is related to that issue : #1662 (closed).

Currently there is : "YYYY-MM-DDTHH-MM-SSZ" that is outputted for the date. With the fix it is for example : "2020-11-30T00:24:55Z".