SACRAMENTO, Calif. — State regulators gave a bleak drought warning Monday to the farms and cities that draw drinking and irrigation water from California’s major rivers: Prepare for mandatory cutbacks. The State Water Resources Control Board announced it was sending letters to approximately 20,000 water right holders — farmers and cities with historical legal claims to river water. Facing a third straight year of drought, the letter says they should expect to stop pulling water in the coming weeks — and even earlier than last year. It wasn’t until last August that the water board ordered thous…