ATLANTA — Paul Johnson is vowing to turn things around after Georgia Tech struggled through its worst season in more than two decades. The Yellow Jackets, touted as an Atlantic Coast Conference championship contender at the beginning of the year, finished a dismal 3-9 campaign with a 13-7 loss to rival Georgia. After his team dropped nine of its last 10 games, the coach had a blunt assessment: “Unacceptable.”