Twenty21, the polished, business-friendly new-American restaurant-lounge in Commerce Square that replaced Cutters in 2002, will close at the end of June to reconceptualize. Owner Sue Mahoney won't say what the new idea is, but hopes to reopen by Labor Day. On the other side of Commerce Square's plaza, where a Marathon Grill was, construction of the casual Pagano's Market continues. Opening seems a few weeks off.