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