Add support for 1.21.4

[Jikoo]

This is the initial copy+paste+compilation fix of the 1.21.3 adapter. I have not yet had time to explore the changes.

TODO for release:

• Check game mode field
• Check player tag deserialization
• Update resource pack versioning if necessary
• Investigate potential for increased compatibility with other resource packs via String-based custom model data
• Fix empty armor slot textures

/e: Numbers are hard.
As always, standard disclaimer about how I have not tested this in game yet. It looks like it should work fine, but if you're downloading a build from an Actions run, please be sure to try it on your test server before pushing to production.

[Jikoo]

If anyone happens to know how to access GUI textures in items, please let me know. The empty slot textures have moved from assets/minecraft/textures/item/empty_armor_slot_<slot> to assets/minecraft/textures/gui/sprites/container/slot/<slot>, and I can't figure out how to get them to load.
To my understanding, the resource location should be relative to assets/minecraft/textures by default with the minecraft key, so minecraft:gui/sprites/container/slot/boots should load assets/minecraft/textures/gui/sprites/container/slot/boots.png.

I will likely resort to just copying the textures, but I'm not happy about it. I'd rather not duplicate resources that technically could be eligible for DMCA.

/e: In case anyone else runs into this problem, it does not appear to be possible. Internally, all references to the textures I want to use are referenced by minecraft:container/slot/<slot> from a different atlas. Since the textures aren't in the blocks atlas at all, they can't be accessed by items.
It might theoretically be possible to bundle symlinks instead of copies of the textures, but I don't know how that would work for cross-system compatibility of the pack. It also would still require the textures to be loaded into memory twice, so it doesn't really save resources.