WASHINGTON, March 23 (Reuters) - U.S. corporate due diligence firm Mintz Group said on Thursday its Beijing office was raided by authorities and five Chinese staff were detained, stoking worry among foreign companies in China just as its capital hosts an international economic forum. News of the raid and detentions comes as Sino-U.S. relations have spiraled downwards following months of diplomatic tensions, including over the U.S. military downing in February of a suspected Chinese spy balloon and a planned U.S. transit next week by the president of Taiwan, the self-governed island China claims as its territory. "We can confirm that...