Additional tweaks for 2.6.9.3-EXP

After further experimentation, dungeon generation still feels off.

As a base, Overworld dungeon spawn rates have been boosted to 150% (excluding ships), then:

Dungeons visible from the surface have been made 150% more common on top of that

Underground-only dungeons have been toned down, relatively.

Additionally, to cover an oversight, Nether Cities can now spawn in the Nether biome in addition to the current valid biomes.