Donald J. Trump and Mitch McConnell haven’t said a word to each other since December 2020. But people close to both men are working behind the scenes to make bygones of the enmity between them and to pave the way for a critical endorsement of the former president by the highest-ranking Republican holdout so far, according to three people familiar with both teams’ perspectives who were not authorized to discuss the situation publicly.Assuming it happens, Mr. McConnell’s endorsement of Mr. Trump would have enormous symbolic value to the former president, giving him the embrace of the…