Add OARS tags to the appdata
Summary: OARS tags allow software centers such as Discover, GNOME Software or the Flathub website to generate content ratings for age etc.. This is pretty useful for an app like gcompris as it can advertise how appropriate it us for users of all ages.
Reviewers: GCompris, jjazeix
Reviewed By: GCompris, jjazeix
Subscribers: jjazeix, KDE Edu
Differential Revision: https://phabricator.kde.org/D12688
(cherry picked from commit 38f1204d5f2952fc512611afd266b36feba3618e)