Re-add the Overkill advancement

The Mace Advancement, "Over-Overkill" currently references an Achievement that no longer exists in Java Edition, making its name confusing. Re-adding Overkill makes the reference make sense, while also perhaps telling the player that there's ways to increase their weapons' damage, serving the purpose that Advancements have (not necessarily goals, but moreso hints for what one can do in the game)