Boston’s St. Patrick’s Day has long been known for a boozy good time. But a growing number of people have found ways to participate in the parades, festivals and banquets without a buzz. They’ve refocused on celebrating Irish heritage and are turning to nonalcoholic beers, mocktails and solely sober spaces. Bar owners and managers in Boston also say most establishments now serve nonalcoholic drinks — a significant shift from decades ago. And it’s not just bars seeing an uptick in a demand for nonalcoholic options. Polling by Gallup shows young adults are drinking less than they were in decades past.