In one of the highest-profile retractions in the history of broadcast news, CBS admitted Monday that documents it used for a story accusing President Bush of receiving preferential treatment during his Vietnam-era service in the National Guard may have been forgeries. ''We should not have used them,'' CBS News President Andrew Heyward said. ``That was a mistake, which we deeply regret.'' CBS anchor Dan Rather, who for the past two weeks claimed that critics of the documents were merely disgruntled Bush supporters, added his own apology. ''If I knew then what I know now, I would not have gone ahead...