Fixes Soul Scythe being able to get to Centcom by moving down on the bottom Z-level (#71171)

## About The Pull Request

`/obj/item/soulscythe/relaymove()` was using `get_step()` which doesn't
understand our multi-z system and was happily trying to move Z - 1 which
is Centcom. I'm still not really sure I understand why move() allowed
the scythe to just move right through the floor in this case, I think
moving to turfs with `density = 0` is also behaving strangely and just
skipping some checks that should keep it from moving through the floor,
but to be honest I don't fully understand the move chain and just
changing to `get_step_multiz()` at least keeps the scythe from going to
Z-levels it shouldn't.
## Why It's Good For The Game

Whilst it is fun for the scythe to go on an adventure to forbidden
Z-levels, admins probably don't appreciate these adventures so much.
## Changelog
🆑 VexingRaven
fix: Soul Scythes can no longer phase through the floor into Centcom.
/🆑

code/modules/mining/lavaland/megafauna_loot.dm

@@ -439,7 +439,7 @@
 		return
 	if(pixel_x != base_pixel_x || pixel_y != base_pixel_y)
 		animate(src, 0.2 SECONDS, pixel_x = base_pixel_y, pixel_y = base_pixel_y, flags = ANIMATION_PARALLEL)
-	Move(get_step(src, direction), direction)
+	Move(get_step_multiz(src, direction), direction)
 	COOLDOWN_START(src, move_cooldown, (direction in GLOB.cardinals) ? 0.1 SECONDS : 0.2 SECONDS)
 
 /obj/item/soulscythe/throw_impact(atom/hit_atom, datum/thrownthing/throwingdatum)