Deprecate itemGroups field

Since 1.19.3, this was only populated when the player opened the
creative menu, and so was useless in survival or multi-player
worlds.

Rather than removing the field entirely (🦑 backwards compatibility), we
replace it with the empty list. We also remove it from the docs, and add
a note explaining what the field used to do.

Closes #1285, albeit in the least satisfactory way possible.

projects/common/src/test/java/dan200/computercraft/TestPlatformHelper.java

@@ -36,7 +36,6 @@ import net.minecraft.world.entity.player.Player;
 import net.minecraft.world.inventory.AbstractContainerMenu;
 import net.minecraft.world.inventory.CraftingContainer;
 import net.minecraft.world.inventory.MenuType;
-import net.minecraft.world.item.CreativeModeTab;
 import net.minecraft.world.item.Item;
 import net.minecraft.world.item.ItemStack;
 import net.minecraft.world.item.crafting.Recipe;
@@ -165,13 +164,6 @@ public class TestPlatformHelper extends AbstractComputerCraftAPI implements Plat
         throw new UnsupportedOperationException("Cannot interact with the world inside tests");
     }
 
-
-    @Nullable
-    @Override
-    public ResourceLocation getCreativeTabId(CreativeModeTab tab) {
-        return null;
-    }
-
     @Override
     public RecipeIngredients getRecipeIngredients() {
         throw new UnsupportedOperationException("Cannot query recipes inside tests");