boards native_sim: Deprecate CONFIG_NATIVE_SIM_NATIVE_POSIX_COMPAT

This option existed only to make the transition from native_posix to
native_sim easier. As native_posix is going to be removed in v4.2
we deprecate this option now, so it will also be removed.

We also switch this option to default to false already now.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>