By Chuck Mikolajczak NEW YORK (Reuters) – U.S. stocks closed out the first quarter on a down note on Thursday with its biggest quarterly decline in two years as concerns persisted about the continuing conflict in Ukraine and its inflationary effect on prices and the Federal Reserve’s response. While optimism about a possible peace deal between Ukraine and Russia helped lift stocks earlier in the week, hopes quickly evaporated and Russia’s President Vladimir Putin threatened on Thursday to halt contracts supplying Europe with a third of its gas unless they are paid in rubles as Ukraine prepared…