By Nandita Bose MUNICH (Reuters) -The United States and its allies will impose “significant and unprecedented” economic measures against Russia if it attacks Ukraine, U.S. Vice President Kamala Harris said on Saturday. “National borders should not be changed by force,” she said at the Munich Security Conference. “I can say with absolute certainty if Russia further invades Ukraine the United States, together with our allies and partners, will impose significant, and unprecedented economic costs,” she said. “We have prepared economic measures that will be swift, severe, and united,” Harris added…