porting idea for worlds before 1.17

Hi Mojang,

my suggestion for porting pre-1.17/snapshot worlds is to replace the existing bedrock with grimstone and generate caves and stuff below y=0 similar to how new chunks are generated. If the world being opened is from a previous version without the new y=-64 bedrock level, use the above suggestion, if it's a new world/chunk just use normal snapshot/1.17+ generation.