By Jeff Mason and Nandita Bose WASHINGTON/ON BOARD AIR FORCE ONE (Reuters) -U.S. President Joe Biden expressed disappointment on Thursday over announced plans by OPEC+ nations to cut oil output and he and officials said the United States was looking at all possible alternatives to keep prices from rising. Despite that move, Biden said he did not regret his summer trip to Saudi Arabia, which he said was focused on the Middle East. OPEC+ agreed to steep oil production cuts on Wednesday, curbing supply in an already tight market and raising the possibility of higher gasoline prices right before t…