TaTe's Inventory Enhancements

Forgot to actually update the upgrade functions.

Custom items now use item_model instead of custom_model_data. If upgrading from 1.21.4, broken items can be fixed by having them in your inventory and running the following command(s):

/function tateinventory:upgrade/1_21_4_to_1_21_5_single (only upgrades for person who runs function)

/function tateinventory:upgrade/1_21_4_to_1_21_5_all (upgrades for all players in server)