Prettier Horses

-support for 1.21.9 and above

-fixed incompatibilities with optifine

-added two new variants for zombie horses

-added two new variants for donkeys

-fixed baby horses having missing or broken leg textures

-unicorns now have a 3% chance of spawning or occur if a horse is spawned in a dark oak forest, or is named unicorn

-fixed pack warning in the resource pack menu