Revert Nether Skeleton Spawning Change from 1.19.3

In the snapshots for 1.19.3, the light level required to prevent skeletons from spawning was reduced to stop them from spawning in Nether Portals. This has caused problems, however. The change now prevents them from spawning within the proximity of Soul Fire sources, meaning that the Soul Sand Valley biome has reduced spawns. Additionally, Soul Torches and Soul Lanterns now prevent spawns. This causes problems because these light sources are often used to light areas whilst keeping spawn rates intact. Instead, the change should be to mark Nether Portals as non-spawnable for the Skeleton and Wither Skeleton mobs.