The longtime artistic director of Providence’s Trinity Repertory theater company plans to step down next fall, the theater company announced Wednesday. Curt Columbus has led the theater group since 2006, directing more than 25 productions. In a statement, Columbus said it is “time for new adventures, for me and for Trinity Repertory Company.”“I firmly believe that every artistic endeavor needs renewal and revitalization,” Columbus said. “While I could stay at Trinity Rep forever, the health of the theater and its artistry depends upon this kind of change.”Trinity Rep says it is conducting a national search for a new artistic director. The company produces several plays each season, including new works, and it received the Tony Award for Outstanding Regional Theater in 1981.