WASHINGTON — President Joe Biden and his likely Republican challenger Donald Trump both head to the U.S.-Mexico border in Texas on Thursday in a sign of how central immigration has become to the 2024 election and how much both candidates want to use it to their advantage. Each has chosen an optimal location from which to underscore his respective points.Biden, who wants to spotlight how Republicans tanked a bipartisan border security deal on Trump’s orders, will go to the Rio Grande Valley…