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

roles/mailman3/defaults/main.yml

@@ -85,6 +85,7 @@ lists:
   arch-wiki-admins:
     advertised: false
     archive_policy: private
+    default_nonmember_action: defer
     display_name: Arch-wiki-admins
     subscription_policy: confirm_then_moderate
   arch-women: