games/openttd: update OpenTTD to version 14.1 (major update)

Users and server operators should read the announcement:
https://www.openttd.org/news/2024/04/13/openttd-14-0

Drop nowadays needless USES+=gnome: as explained by tobik@,
it was required for INSTALLS_ICONS which only did something
when `gnome.mk' was loaded, but this knob is long gone.