San Francisco will ban the use of natural gas in new buildings starting next year, becoming the latest city in California to clamp down on the heating and cooking fuel because of climate concerns. The measure will require all-electric construction for buildings — with exceptions for restaurants — starting in June 2021, according to an ordinance passed late Tuesday by the San Francisco Board of Supervisors. It’s the strictest natural gas prohibition passed by a big city so far in California, according to the Sierra Club, an environmental advocacy group.