thegamesdb: Update the Dreamcast MIME type

The application/x-dc-rom MIME type was removed as it seems to come out
of nowhere, the newly added application/x-dreamcast-rom type can replace
it here and in GNOME Games.

See:
- https://gitlab.freedesktop.org/xdg/shared-mime-info/-/merge_requests/88
- https://gitlab.freedesktop.org/xdg/shared-mime-info/-/merge_requests/89
- https://gitlab.gnome.org/GNOME/gnome-games/-/merge_requests/430
2 jobs for dreamcast in 7 minutes and 3 seconds (queued for 1 second)