AUBURN, Maine — A 150-year-old institution here has major growth plans, including a new $15 million fitness center in Bates Mill No. 5 in Lewiston and a $4.6 million Outdoor Learning and Education Center in Auburn. “How can the YMCA stay relevant?” Steve Wallace, CEO of the YMCA of Auburn-Lewiston, said to a reporter during a recent tour of the YMCA’s main building on Turner Street in Auburn. Wallace, a former marine wearing a tracksuit, exudes enthusiasm as he shakes hands with a man on a stationary bike and yells “hello” to professional wrestler Tony Atlas, who is working out in the weight room.