Remember the old 1970s sitcom “Welcome Back, Kotter”? It aired on ABC from 1975 to 1979. The president of the United States throughout half that period was the infamous peanut-farmer-turned-worst-president-ever Jimmy Carter. Perfect timing. Because 2021 is clearly the new 1977. The new version of that sitcom is “Welcome Back, Carter. Except Add Dementia.” It …