Use upstream app ID
This should make the flatpak correctly show up in gnome-software, fixing
io.github.EndlessSky.endless-sky (flatpak) vs
io.github.endless_sky.endless_sky (appdata) mismatch.
Sadly this means it won't match with flathub's ID, but they are planning
on changing it to follow upstream, so it should get sorted out in the
future.
See https://github.com/flathub/io.github.EndlessSky.endless-sky/issues/2