By Michelle Nichols UNITED NATIONS (Reuters) – Britain’s Prince Harry spoke on Monday at the United Nations about seeking insight from the late Nelson Mandela during a time of global uncertainty and urged countries to take action on climate change. Accompanied by his wife Meghan, Duchess of Sussex, the couple held hands as they walked into U.N. headquarters in New York City to mark Nelson Mandela International Day, held annually on the former South African president’s birthday. In the U.N. General Assembly hall, Harry spoke about the threats from Russia’s invasion of Ukraine, what he called th…