Various nether changes are leaking into old base_game worlds

Partner Rank: 3

Verification builds: 1.16.61
Regression builds: 1.14.6005

Summary: This is a repost of MCPE-66357 for MCM. The new nether biomes and structures are generating even in old base_game worlds.

Impact: Any old content that provides access to a vanilla world is full of "update game" blocks in new terrain/the nether. For example this affects our map ANIMALS

Repro Steps:

1. Find any world with base_game version 1.13.
2. Enter the nether.

Expected Results: Pre-nether update nether.

Observed Results: Tons of "update game" blocks all over the place.

Screenshots/Videos attached: Yes