By Toby Melville NEW YORK (Reuters) -Britain’s Liz Truss will use her first international trip as prime minister to promise billions of pounds of more support for Ukraine next year, her office said on Tuesday ahead of Truss’ speech at the United Nations this week. In addition to addressing the U.N. on Wednesday, Truss hopes the trip to New York will reinvigorate the so-called special relationship with the United States after ties soured over post-Brexit trade. Truss will pledge at the U.N. summit to meet or exceed in 2023 the 2.3 billion pounds ($2.6 billion) of military aid spent on Ukraine i…