(Reuters) – About 1,000 civilians remained trapped at a steel plant where Ukrainian forces are making their last stand in the besieged city of Mariupol, President Volodymyr Zelenskiy said, after a Russian ultimatum to surrender or die expired on Wednesday. More than five million people have now fled Ukraine, the United Nations refugee agency said. WIDER IMPACT * Top finance officials from Britain, the United States and Canada walked out of Wednesday’s meeting of finance officials from the world’s top 20 economies as Russian representatives spoke. * President Joe Biden will host U.S. military l…