SPRINGFIELD — This Saturday marks Justice Robert R. Thomas’ last day on the Illinois Supreme Court.Next week, the high court will welcome a new member, Justice Michael J. Burke, to the bench. Because of the timing of Thomas’ retirement announcement so close to the upcoming primary elections, the new Justice Burke will hold onto the seat through late 2022.Thomas is set to join the plaintiff’s firm Power Rogers LLP. His judicial retirement announcement didn’t offer details about the timing of …