Russia freed WNBA star Brittney Griner on Thursday in a dramatic high-level prisoner exchange, with the US releasing notorious Russian arms dealer Viktor Bout, American officials said. The swap, at a time of heightened tensions over Ukraine, achieved a top goal for President Joe Biden, but carried a heavy price — and left behind an American jailed for nearly four years in Russia. The deal, the second such exchange in eight months with Russia, procured the release of the most prominent American detained abroad. Griner is a two-time Olympic gold medalist whose months-long imprisonment on drug ch…