Fixed pregen shutdown scheduling

- Scheduler admission avoids the plugin manager monitor
- Saving callbacks no longer block the joined pregenerator during disable

core/src/main/java/art/arcane/iris/util/common/scheduling/J.java

@@ -772,7 +772,7 @@ public class J {
     }
 
     private static boolean isPluginEnabled() {
-        return usesBukkitScheduler() && BukkitPlatform.hasPlugin() && Bukkit.getPluginManager().isPluginEnabled(BukkitPlatform.plugin());
+        return usesBukkitScheduler() && BukkitPlatform.hasPlugin() && BukkitPlatform.plugin().isEnabled();
     }
 
     private static boolean canSchedule() {