Around the country, countless Americans are suffering a very particular type of Covid grief — a mixture of anger, sorrow and shame that comes with losing a loved one who has consumed social media falsehoods. On Tuesday, in what was likely his last appearance in the White House briefing room before he retires from government service at the end of the year, Dr. Anthony S. Fauci, President Biden’s chief medical adviser, pleaded with Americans to speak out against scientific misinformation.