After three superbly savage seasons, “The Boys” has graduated to the spinoffs stage, first with the animated shorts “Diabolical” and now “Gen V,” a supes-in-college series that’s every bit as nasty – in mostly good ways – as the flagship series. Cleverly drafting off the anchor show, this coming-of-age superhero satire in some ways feels like what “The New Mutants” should have been.