U.S. President Joe Biden abruptly canceled a scheduled speech in Las Vegas after testing positive for COVID-19, the White House said, adding that the president was experiencing only “mild” symptoms. Biden, who is 81, was scheduled to deliver remarks at the UnidosUS Annual Conference in Las Vegas on Wednesday afternoon, but the speech was canceled at 3 p.m., when the audience had already been waiting for 90 minutes. “Regrettably, I was just on the phone with President Biden and he shared his deep disappointment at not being able to join us this afternoon,” Janet Murgua, the president of UnidosU…