Twenty-three years after its American premiere at the Brooklyn Academy of Music, Mark Morris’s “The Hard Nut” returns to the venue through December 20 as a modern classic. That status was not entirely assured when this cheeky re-imagining of the ballet set to Tchaikovsky’s beloved “The Nutcracker Suite” first electrified audiences. “It’s deepened over the years,” says Morris now. “People who saw it as kids are now bringing their own kids.”