Prince William cozied up to pop princess Taylor Swift knowing all too well his growing friendship with the Cruel Summer singer would set off the snubbed Duke and Duchess of Sussex — and palace spies say the unwitting superstar is caught in the middle of a royal civil war, RadarOnline.com has learned. William, 42, beamed backstage in London as Taylor and her football player beau, Travis Kelce, snapped after-concert pics with the future king and his two oldest kids, Prince George, 10, and Princess Charlotte, 9. The “Shake It Off” songbird even shared one selfie online and wished Wills a happy bi…