Discovered while testing MC-270989.

The bug

If a 1.12.2 world containing snow golem spawn eggs or iron golem spawn eggs is upgraded to 1.20.5, they do not become the proper IDs for these spawn eggs, and instead become pig spawn eggs.

How to reproduce

A 1.12.2 world with many spawn eggs is attached to this ticket.

1. Load this world in 1.12.2
2. Check the spawn eggs in the leftmost chest
3. Upgrade to the latest development version
4. Check those eggs again

Expected results

The iron golem and snow golem spawn eggs would become proper iron golem and snow golem spawn eggs.

Actual results

They instead become pig spawn eggs with extra components.