refactor: remove fine-grained biome controls and adjust biome handling

Removed the deprecated `doFineGrainedBiomes` option from `TerraMinestomWorldBuilder` and associated classes. Updated biome management to use `IdentityHashMap` and added additional tracking for created biomes. Upgraded Minestom version to `1_21_6-a40d7115d4`.

buildSrc/src/main/kotlin/Versions.kt

@@ -89,6 +89,6 @@ object Versions {
     }
     
     object Minestom {
-        const val minestom = "1_21_6-c3ccee696b"
+        const val minestom = "1_21_6-a40d7115d4"
     }
 }