create alias arms_under_breasts -> breast_hold
The definition of breast hold covers what 'arms under breasts' conceptually suggests.
Found a bunch of posts tagged with arms under breasts and fixed them, but I want to future proof the resurrection of the tag since it isn't intuitive for breast hold to be used for this purpose
EDIT: The tag alias breast_hold -> arms_under_breasts (forum #149131) has been rejected by @Hillside_Moose.
Updated by DanbooruBot