Fix the effective level difficulty of all Sokoban levels to either the level past the one with the stairs, or the level with the stairs, or a constant 9.

It’s a common refrain that Sokoban was too easy in 3.4.3 with its easier difficulty as you progress through it, and that it was too hard in 3.6 with its harder difficulty. Fixing it at a constant difficulty is a popular middle ground.