BRAINTREE, Mass.—A former flight attendant who lost several colleagues when United Flight 175 was flown into the World Trade Center’s south tower in New York on Sept. 11, 2001, is honoring his friends on the 20th anniversary of the terrorist attacks by pushing an airline beverage cart from Boston to ground zero. Pushing a cart is familiar to Paul Veneto, 62, who spent 30 years as a flight attendant for five different airlines. Flight 175 was his regularly scheduled flight. He had flown into Boston the night before and Sept. 11 was his day off, he told The Patriot Ledger of Quincy. He was helping a friend build a concrete wall at the time of the attacks, and didn’t even know at first the second plane to hit the towers was his regular flight. When he found out, “I was in shock,” he said. “I wanted revenge. I was angry …