Some hoglin stable stair structures do not generate

The affected structures are:

• stairs/stairs_0_mirrored
  • Note that this structure is separate from starting_pieces/stairs_0_mirrored. They have different structures and the latter does indeed generate.
• stairs/stairs_2
• stairs/stairs_3

None of these stairs are in any hoglin stable jigsaw block pool. They would generate just fine if added to their respective pools being mirrored_starting_pieces for the first and stairs for the second and thirds. They have jigsaw blocks set up similarly to other stairs in these pools.

Relates to MC-183594