Limit how many slimes can spawn in one chunk

[MilesBHuff]

In 1.21, infinite slimes can be spawned extremely cheaply, and this can easily be scaled to take down a vanilla server at worst, or to generate boatloads of infinite slime and experience for free at best. So we have a DoS and a balance issue in one.

Paper etc have mechanisms to limit mob spawns, and we are already using these; nevertheless, it is critical that we test these when upgrading to 1.21.