The 60-story, $700 million Millennium Tower Boston, which is rising next door to the former home of Filene’s department store in Downtown Crossing, held a topping-off ceremony Thursday. Mayor Martin J. Walsh and about 450 construction workers from the project attended. At a topping-off ceremony, a traditional final beam is raised, along with a Christmas tree, to signify that the structure has reached its final height. Residents are scheduled to move in to the building in the summer of 2016.