During his final year in office, former President Joseph R. Biden Jr. moved to downgrade marijuana from the government’s most restrictive class of drugs. The change would not legalize recreational cannabis under federal law, but would remove it from a category that includes heroin and make it easier for scientists to study marijuana’s therapeutic uses. Donald J. Trump, then a candidate for president, endorsed the idea last September, shortly before the presidential election, saying it stood to “unlock the medical uses of marijuana.” But a few months into Mr. Trump’s new term in the White House, the push to move cannabis to a less restrictive category has effectively ground to a halt.