W50 sprinter Colleen Barney has been posting Facebook photos of herself training at a gym — hinting that she’s prepping for “American Ninja Warrior.” Last week, she made it official. She aims to be the best over-50 woman ever to appear on the reality show when she tackles the L.A. Universal Studios course March 6. She’s not sure if she’s the oldest woman ever invited, “but I do hope to make it further than any woman over 50 has gone (right now that means I have to complete at least the first two obstacles)!” Colleen — already a certified Super…