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.
Added additional spawning biomes to the Nether City for 2.6.9.4-EXP

This now lets it spawn anywhere in the Nether provided the conditions are met
Merge tag 'tags/2.6.9.4' into serverExp