make underground inner walls same as outer

[marilynias]

Summary

None

Purpose of change

#77822 made some shelter walls parametrized, but not all. #78230 fixes the surface level of them.
The underground level currently has concrete outer walls and parametrized (different from surface) inner walls. This PR seeks to make them the same
I made this a different PR since I can see an argument to be made for making all basement walls parametrized (same as above ground) or make them all concrete. My preference is all concrete, because underground being eg. wooden doesn't fit imo.

Describe the solution

replace inner walls of all shelter_under mapgen with concrete walls (same as outer walls)

Describe alternatives you've considered

Make the outer walls parametrized

Testing

spawn in some shelters and go underground

Additional context

before:

after:

parametrized underground, imo inferior solution:

[Maleclypse]

Basements interior walls in New England are made of the parametrized materials. This shelter level is built like a basement not like a dungeon.