Anybody who’s seen “Love Story: John F. Kennedy Jr. and Carolyn Bessette” knows what it should really be called: “Love Letter.” I can hardly recall a kinder, gentler or more appealing depiction of the Kennedys ever onscreen. Yet Jack Schlossberg, President John F. Kennedy’s attention-loving grandson, is so enraged by the popular new FX miniseries that last week he took to TV to condemn it while running for office. On CBS Sunday Morning, the Democratic candidate for New York’s 12th congressional