Fabric explains it a bit here, Forge needs to provide a guide for 1.21.3 since the examples are Fabric specific: https://fabricmc.net/2024/10/14/1212.html
EDIT: This seems to work but I don't know if this is the proper way to do it. Truly an annoying change especially if you have lots of items and blocks:
Old:
public static final RegistryObject<Item> ITEM_EGG = ITEMS.register("egg", () -> new Item(new Item.Properties()));
New:
ITEM_EGG = ITEMS.register("egg", () -> new Item(new Item.Properties().useItemDescriptionPrefix().setId(ResourceKey.create(Registries.ITEM, ResourceLocation.parse("modid:egg")))));
Neo also has more on the topic: https://neoforged.net/news/21.2release/
