SOONER or later every fairy tale reaches its happily ever after. For “Shrek,” the long-running animated fantasy franchise about a curmudgeonly ogre (voiced by Mike Myers), his human-turned-ogre wife, Fiona (Cameron Diaz), and his pals Donkey (Eddie Murphy) and Puss in Boots (Antonio Banderas), that time is now. On Friday, DreamWorks Animation and Paramount will release “Shrek Forever After,” the fourth — and, the studios say, final — installment in the series, in which the titular green guy is thrown into an alternate version of his Far, Far Away kingdom where he never existed and life turned out much differently for his pals. Before the storybook shuts for good, Dave Itzkoff asked some of the “Shrek” stars about the series and what its ending means to them