Oli Higgins, who was a critical member of Brewer High School’s varsity boys basketball team, is leaving to play for a top basketball prep school. Higgins announced on his Instagram account Thursday that he would be attending the Newman School in Boston. He will also be reclassifying to graduate in the class of 2029 — adding an extra year to his high school career. Higgins, who is 6-foot-10, according to his Instagram, was one of Maine’s best hig…