SPRINGFIELD — Illinois Supreme Court Justice Robert R. Thomas, 67, is retiring after a 20-year tenure at the high court.His retirement is effective Feb. 29, according to a court-issued news release Monday.Thomas, a former Chicago Bears place kicker, will join Power Rogers LLP, the powerful plaintiffs’ firm where his son, Jonathan M. Thomas, currently practices.“It has been a great honor and privilege to have served on the Illinois Supreme Court as well as on the appellate and circuit courts over the past …