Prior to this commit, the lister assumed authentication was required while it exists
public gogs instances which do not require it.
This also updates documentation to mention the usual api location to check when actually
triggering a listing.
Co-authored with Antoine Lambert (@anlambert) <firstname.lastname@example.org>
Related to infra/sysadm-environment#4644
Related to https://gitlab.softwareheritage.org/infra/sysadm-environment/-/issues/4644#note_12601