All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu`alayhi wa sallam, is His slave and Messenger.
What you mentioned about the way of raising the chickens does not affect the permissibility of their meat as long as they were slaughtered according to Islamic law, and giving them antibiotics does not mean that they are forbidden either.
Allah knows best.