Caves Update and Old Worlds - Regenerate for y < 5

To support old worlds in the Caves & Cliffs Update and to prevent Chunk Borders that could cause people dig into the void, Chunks from old worlds should regenerate on Y < 5.

This will not interfere with any player builds unless the player used an exploit to remove Bedrock on the bottom of the overworld - and even in this case, chances are that the player did this in order to get higher spawn rates, which won't be the case after the Update anymore anyway, because the most bottom block is now 64 blocks further down (which he can't reach if the bedrock from the old generation is still there).