“Cinderella,” unsurprisingly, are its cast and Lloyd Webber’s heart-racing ballads sung on a bare stage.Cinderella, Georgina Onuorah (she is the alternate, the role is normally played by Carrie Hope Fletcher), makes a striking West End debut with modern pop flair in a song called “I Know I Have a Heart.” She sits, shattered after the ball, on the center of the stage in a big white gown and powerfully wails her pain right to us.As her Prince Sebastian, a nontraditionally geeky heir, talented