mailman3: allow everyone to post to the arch-wiki-admins mailing list

The mailing list is used for non-public communication with users, so everyone needs be able to post to it.
It is also the assigned email address of the ArchWiki user "WikiSysop".
See https://wiki.archlinux.org/title/ArchWiki:Maintenance_Team#Who,_when_and_how_to_contact