PHILADELPHIA — The Philly boos came out when Shake Milton lost the ball on a second-quarter turnover. The reaction shifted to an exasperated groan when Jaylen Brown hit a three-pointer that pushed the Boston Celtics’ lead to 24 with less than seven minutes to play before halftime. Then those fans chanted their displeasure at the officials when Joel Embiid was called for a foul at the 5:15 mark of that period. And that was before things turned even uglier in the second half. A disastrous on-court performance put a damper on what began as a celebratory day for the 76ers, who officially welcomed …