appdata: Add release tag

The appdata validation is now more strict and has a
mandatory requirement of the "release" tag [1]

[1] https://github.com/hughsie/appstream-glib/pull/296
1 job for master in 26 minutes and 55 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #260076
flatpak-documents

00:26:55