Cushman & Wakefield has been hit with a double whammy. First, the global real estate giant was the victim of a cyberattack — and now it faces a proposed class action for allegedly failing to protect former and current clients’ personal information. Commercial tenant Michelle Milewski, of Illinois, filed the proposed class action last Friday, just days after hacker groups notorious “ShinyHunters” and “Qilin,” a Russian-speaking cybercrime gang, infiltrated the firm’s systems, according to the federal complaint obtained by The Post from the Southern District of New York. Cushman & Wakefield, a firm known around the globe, was the victim of a data breach last week. Court Documents Two groups, “ShinyHunters” and “Qilin,” were behind the cyberattack.…