it will be available on all FortiGate versions, I can speak only from 5.2 and newer.
On CLI you will be able to see/set it here:
config system global
set admin-server-cert "mywebservercert"
On GUI, lacking the description, it is supposed to be here:
GUI > System (on global VDOM, if you have VDOMs) > Settings
And there "HTTPS server certificate"
If there is no "Settings", you are in a VDOM, you will have to switch to global.
An indicator, at least for me, whether I am in the VDOM is on the browser address bar, it says vdom=<vdomname>, for example