cherry-pick from github.com/minetest/minetest_game> Stairs/default: Make...

cherry-pick from github.com/minetest/minetest_game> Stairs/default: Make sandstone(brick) groups consistent

Sandstone is crumbly = 1 cracky = 3 to be slowly diggable by hand
Sandstonebrick(stair/slab) is cracky = 2