By Steve Holland WASHINGTON (Reuters) – A slow-rolling staff shakeup is under way at the White House as President Joe Biden girds for battle in the November midterm elections, where he hopes to help his Democrats hang on to control of the U.S. Congress. Cedric Richmond, a former Democratic member of the U.S. House of Representatives from Louisiana who as the director of the White House office of Public Engagement has been a top adviser to Biden since the president’s campaign for office, will depart to the private sector, a person briefed on the move said. Richmond will also become a senior adv…