Alex Dancyg, Yoram Metzger, Haim Perry, Yagev Buchshtav, Nadav Popplewell, and Avraham Munder were fatally shot in February by their Hamas captors in a tunnel in Khan Younis following an IDF strike in the area; Report finds IDF was unaware of hostages' location when approving the airstrike, which caused toxic gases to seep into the tunnel killing the terrorists