component/postfix: version up 3.7.3

3.7.3 is an "urgent" new release as announced in
https://www.postfix.org/announcements/postfix-3.7.3.html
the old 3.7.2 version seem to have bee removed from mirrors
(see http://postfix.mirrors.ovh.net/postfix-release/official/ for a
mirror with directory index) and was not in shacache.