Un-break for versions <1.21.3

[SirYwell]

Overview

Fixes #2987

Description

Compile against 1.21 again to avoid bytecode incompatible with version <1.21.3. Also apply an upstream patch and remove now-unneeded code.

The upstream patch initially included doesn't support custom biomes pre 1.21.3, so we need to revisit that separately.

### Submitter Checklist
- [x] Make sure you are opening from a topic branch (**/feature/fix/docs/ branch** (right side)) and not your main branch.
- [x] Ensure that the pull request title represents the desired changelog entry.
- [x] New public fields and methods are annotated with `@since TODO`.
- [x] I read and followed the [contribution guidelines](https://github.com/IntellectualSites/.github/blob/main/CONTRIBUTING.md).