Dungeon rooms generated below y=0 use cobblestone

Issue: Dungeons generated below y-level 0, where deepslate starts have walls and floors that use cobblestone making them look out of place.

Intended Results: Dungeons generated below y-level 0 should use cobbled deepslate or deepslate.

Steps to reproduce:

1. Create a new world in 1.18 pre-release 2
2. Enter spectator mode and locate a dungeon room generated below y-level 0

Seed used in screenshots below: -2376554508837763884
Coordinates: 595, -25, -2600