gogs/lister: Allow public gogs instance listing

Prior to this commit, the lister assumed authentication was required. It exists public
gogs instances which do not require it.

This also updates documentation to mention the usual api location. This is useful when
people wants to actually trigger a listing as a pre-check flight.

This drops repetitive instruction in the gitea lister as well.

Co-authored with Antoine Lambert (@anlambert) <anlambert@softwareheritage.org>.

Related to infra/sysadm-environment#4644