“I’m an acquired taste,” sings Nanki-Poo’s would-be bride, Katisha, “and I’m educating his palate.” The same might be said of the light operas of Gilbert and Sullivan, whose oh-so-English-whimsy The Mikado receives an invigorating American update in Theatre Under the Stars’ current production of Hot Mikado. This big-band, neo-Mikado is…