Fix command pool submitting on shutdown (#8272)

patches/server/Async-command-map-building.patch

@@ -28,7 +28,8 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +        new com.google.common.util.concurrent.ThreadFactoryBuilder()
 +            .setNameFormat("Paper Async Command Builder Thread Pool - %1$d")
 +            .setUncaughtExceptionHandler(new net.minecraft.DefaultUncaughtExceptionHandlerWithName(net.minecraft.server.MinecraftServer.LOGGER))
-+            .build()
++            .build(),
++        new java.util.concurrent.ThreadPoolExecutor.DiscardPolicy()
 +    );
 +
 +    private void sendAsync(ServerPlayer player) {