Use Mob interface for on-spawn targeting.

Since 1.13, we've had the Mob interface injected between Creature and
LivingEntity, and it contains the targeting methods we use. Slimes were
never Creatures, but they _are_ Mobs, so by swapping this interface out,
we effectively patch up functionality that's been broken since the very
beginning. Hopefully server owners don't actually depend on this piece
of functionality being broken...

src/main/java/com/garbagemule/MobArena/waves/MACreature.java

@@ -7,11 +7,11 @@ import org.bukkit.DyeColor;
 import org.bukkit.Location;
 import org.bukkit.World;
 import org.bukkit.entity.Bee;
-import org.bukkit.entity.Creature;
 import org.bukkit.entity.Creeper;
 import org.bukkit.entity.EntityType;
 import org.bukkit.entity.Hoglin;
 import org.bukkit.entity.LivingEntity;
+import org.bukkit.entity.Mob;
 import org.bukkit.entity.PigZombie;
 import org.bukkit.entity.PiglinAbstract;
 import org.bukkit.entity.Rabbit;
@@ -181,9 +181,9 @@ public class MACreature {
                 break;
         }
 
-        if (e instanceof Creature) {
-            Creature c = (Creature) e;
-            c.setTarget(WaveUtils.getClosestPlayer(arena, e));
+        if (e instanceof Mob) {
+            Mob m = (Mob) e;
+            m.setTarget(WaveUtils.getClosestPlayer(arena, e));
         }
 
         return e;