Only require <release> for validate, but not validate-relax
Only require <release> for validate, but not validate-relax

The plan was to require <release> for validate, so that we would be able
to require a version number for flathub, but it accidentally got enabled
for validate-relax as well.

hughsie/appstream-glib,hughsie/appstream-glib,hughsie/appstream-glib