San Francisco health officials have set an ambitious goal of vaccinating all 900,000 people who live and work in the city by the end of June, and took a step toward that goal Friday by opening a mass vaccination center at City College of San Francisco. The opening followed a hearing called by Supervisor Matt Haney earlier this week to answer outstanding questions about the city’s vaccination plan, where health officials and several representatives of private health care providers presented their plans to the supervisors. The main problem, they said, was not having access to enough supply of the vaccines.