By Andrew MacAskill, Kylie MacLellan and David Milliken LONDON (Reuters) -British foreign minister Liz Truss, the frontrunner to replace Boris Johnson as prime minister, was forced to backtrack on one of her most striking pledges a day after announcing it following a backlash from fellow Conservatives and opposition parties. In the first big misstep of her campaign, Truss set out plans to save billions of pounds a year in government spending in a pledge opponents said would require cutting the pay of public sector workers, including nurses and teachers, outside of the wealthy southeast of Engl…