The Boston Celtics are kicking off the offseason sooner than many expected. The Eastern Conference’s No. 2 seed suffered a shocking upset at the hands of the Philadelphia 76ers in the first round of the playoffs, sending Boston home in a grueling seven-game series. After a brief postseason run, Brad Stevens and company will hit the drawing board with a major target in the team’s crosshairs.  The Celtics made a strong push for two-time MVP and Milwaukee Bucks superstar Giannis Antetokounmpo ahead of February’s trade deadline, but the future Hall of Famer remained with the team through the regular season. Entering the offseason, a trade to move Antetokounmpo from Milwaukee appears imminent, and the Celtics are expected to pick up where they left off in their pursuit of the 10-time All-Star.  Antetokounmpo’s relationship with the Bucks has become mightily strained over the past 12 months, with the two sides meeting to discuss his future on several occasions during that span.…