Fortresses can generate in nether forests

Discovered while testing MC-239041.

Fortresses can generate in crimson forests and warped forests. That was never the case, or intended to happen.

How to reproduce:

1. Create a buffet world with the crimson forest or warped forest biome.
2. /locate fortress
3. Look in the chat.

Before anybody asks, no, bastions don't generate in basalt deltas.