By Ted Hesson WASHINGTON (Reuters) – U.S. border authorities arrested 210,000 migrants attempting to cross the border with Mexico in March, the highest monthly total in two decades and underscoring challenges in the coming months for U.S. President Joe Biden. The March total is a 24% increase from the same month a year earlier, when 169,000 migrants were picked up at the border, the start of a rise in migration that left thousands unaccompanied children stuck in crowded border patrol stations for days while they awaited placement in overwhelmed government-run shelters. Biden, a Democrat who to…