By Hilary Russ NEW YORK (Reuters) -Starbucks Corp will pause billions of dollars of stock buybacks, longtime former chief executive Howard Schultz said on Monday as he returns this week to lead the global coffee chain for the third time. Effective immediately, the suspension will allow Starbucks to invest more in employees and stores, Schultz said in a letter to Starbucks workers, customers and shareholders at a time when the company faces growing unionization of its U.S. workforce. Shares fell more than 5% after Wall Street opened. Starbucks spent nearly $12 billion in fiscal 2019 and 2020 co…