gogs/lister: Allow public gogs instance listing
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) anlambert@softwareheritage.org
Related to infra/sysadm-environment#4644 Related to https://gitlab.softwareheritage.org/infra/sysadm-environment/-/issues/4644#note_12601
Migrated from D8752 (view on Phabricator)