In five seasons of the HBO series “Girls,” Lena Dunham has gotten a lot of mileage out of her twenties. But she and the rest of the show’s cast have grown up, and she’s moving on to other projects. Last year, she and Jenni Konner launched the Web site and newsletter Lenny Letter, and she’s currently finishing a collection of short stories. Hours before her thirtieth-birthday party, she sat down with David Remnick to discuss her plans for the future, what it’s like being a Hillary Clinton diehard when everyone around you is feeling the Bern, and why the sixth season of “Girls” will be the last.