MIAMI — The 76ers keep saying everything will be fine. Even when they were decimated by a COVID-19 outbreak and injuries, the Sixers had one message: “For us, it’s the next man up mentality and being locked in on the game plan.” They said it confidently, and we’re about to find out if it’s true when the stakes are high. The Eastern Conference’s fourth-seeded Sixers face the top-seeded Miami Heat in the second round of the playoffs. Game 1 of the best-of-seven series is set for 7:30 p.m. Monday at the FTX Arena. The Sixers are heavy underdogs despite tying the season series 2-2. That’s because …