WASHINGTON—Production at U.S. factories increased to its highest level in nearly three years in November as output rose across the board, providing a powerful boost to economy as the year ends. The manufacturing output index climbed 0.7 percent last month to 100.6, the highest level since January 2019, the Federal Reserve said on Thursday. That followed a 1.4 percent rebound in October. Economists polled by Reuters had forecast factory production rising 0.7 percent. Output increased 4.6 percent compared to November 2020. Manufacturing, which accounts for 12 percent of the U.S. economy, is being supported by strong demand for goods even as spending starts to revert back to services. Inventories at businesses are also extremely lean. But strained supply chains because of the COVID-19 pandemic are a constrain. Spending shifted to goods from services over the course of the COVID-19 pandemic, straining global supply chains. Production at auto plants rose 2.2 …