It’s been happening for a while now. Brooklyn — once a beautiful, messy mix of cultures and communities shaped through a long history of migrations — is being shrink-wrapped once again into a neat bite-sized stereotype. If you saw Catey Shaw’s popular new anthem “Brooklyn Girls,” you’d know the new Brooklyn. Apparently, it is no longer the epic and incomprehensible mass that stretches from Sheepshead Bay up to the Pulaski Bridge. It is the L-Train, and any place where young white people can dress like they’re in a circus and not be mocked openly on the street.