You don’t have to have been alive in 1998 to instantly recognize that jangly mandolin, swelling strings, and those crisp acoustic chords. The Goo Goo Dolls’ “Iris,” a single written for the late-’90s Nic Cage-Meg Ryan cryfest City Of Angels, continues its ubiquity, even 20 years after it first dominated the airwaves. Turn on any soft-rock station, hit the grocery store, or visit a dentist’s office — you might just hear lead singer John Rzeznik’s gravelly croon.And if it’s not “Iris,” it could be one of a string of other Clinton-era classics, many of which show up on the Buffalo group’s sixth,