By Jonathan Stempel (Reuters) – Warren Buffett is this year bringing back the annual extravaganza for shareholders of his Berkshire Hathaway Inc that he calls “Woodstock for Capitalists.” But the 91-year-old billionaire has a cautionary message: If you want to attend, get vaccinated against COVID-19. Proof of vaccination will be required to attend Berkshire’s annual meeting in a downtown Omaha, Nebraska arena on April 30, and to shop from Berkshire-owned businesses in an adjacent exhibit hall. The meeting is the centerpiece of three days of events across Omaha including shopping discounts, an …