Use Overworld cave settings for Cave Ages

docs/PORTING.md

@@ -317,6 +317,12 @@ Void profiles retain their profile-specific biome sources. Legacy's scaled,
 tiled, and grid layouts require dedicated modern biome sources and remain the
 next distribution work.
 
+Version 0.2.21.1 changes Cave Ages from Nether noise settings to Minecraft's
+cave noise settings. Caves remain enclosed, but use stone, water, and
+Overworld-like surfaces together with the persisted Native or Single biome
+distribution. This prevents Nether terrain generation from leaking into newly
+generated Cave Age chunks.
+
 Version 0.2.14.0 restores the legacy Linking and Descriptive Book artwork for
 inventory items and gives each book its matching open-book texture when placed
 on a Book Stand. Return Linking Books created inside another Age also retain and