Nether decorations can be generated on the original bedrock layer

The bug

Nether decorations can be generated on the original bedrock layer.

Steps to reproduce

• Go over the nether ceiling.

  /execute in minecraft:the_nether run tp @s ~ 140 ~
  

• Go to these three biomes:

  /locatebiome minecraft:basalt_deltas
  /locatebiome minecraft:crimson_forest
  /locatebiome minecraft:warped_forest