New York (AFP) – Bill Russell, the cornerstone of a Boston Celtics dynasty that won 11 NBA titles and a powerful voice for social justice, died Sunday at the age of 88, his family said. “Bill Russell, the most prolific winner in American sports history, passed away peacefully today at age 88, with his wife, Jeannine, by his side,” said a statement posted on Russell’s Twitter page. US President Joe Biden and former President Barack Obama — who awarded Russell the Presidential Medal of Freedom in 2011 — were among those who paid tribute to Russell’s contributions on and off the court. “The pro…