SERIES FINALE: This Nickelodeon sitcom, starring Miranda Cosgrove, has been so popular with tweens that Michelle Obama, a fan of the show because of her daughters, guest starred in one episode. (She’s not the first First Lady to take her message to a sitcom, either: Nancy Reagan, pushing her “Just Say No” anti-drug campaign in the 1980s, appeared on Diff’rent Strokes opposite Gary Coleman. As they say: Diff’rent Strokes for different folks.) Anyway, iCarly ends its