Memory Leak: `ServerLifecycleHooks` static reference to `MinecraftServer`

[JoshieGemFinder]

The ServerLifecycleHooks class contains a static reference to MinecraftServer in the form of private static MinecraftServer currentServer

This field is not set to null when the singleplayer integrated server is closed, causing the now-closed currentServer to leak.

To fix this memory leak, either:

• bind to ServerLifecycleEvents.SERVER_STOPPED and set currentServer to null
• replace the currentServer field with a WeakReference