alias arthropod_legs -> arthropod_limbs

BUR #8690 has been approved by @evazion.

create alias arthropod_legs -> arthropod_limbs
create alias insect_arms -> arthropod_limbs
create alias spider_arms -> arthropod_limbs

The tag can also apply to arms, so it makes sense imo to generalize it.

The bulk update request #8690 (forum #204603) has been approved by @evazion.