By Trevor Hunnicutt and Jeff Mason WASHINGTON (Reuters) -President Joe Biden on Thursday commemorated the COVID-19 deaths of 1 million people in the United States, marking what he called “a tragic milestone” and urging Americans to “remain vigilant” during the ongoing pandemic. In a statement, Biden acknowledged the impact of the deaths on families left behind and urged the country not to “grow numb to such sorrow.” The United States on Wednesday reached more than 1 million COVID-19 deaths, according to a Reuters tally, crossing a once-unthinkable milestone about two years after the first case…