Fix Elixir 1.18 "clause will never match" warnings

[justinludwig]

Fixes warnings like the following by providing a hint to the Elixir compiler about the result type of rule blocks:

    warning: the following clause will never match:

        {:ok, description}

    because it attempts to match on the result of:

        case actor.admin? do
          x when x === false or x === nil -> :next
          _ -> :ok
        end

    which has type:

        dynamic(:next or :ok)

    typing violation found at:
    │
 51 │     rule [:update, :delete],
    │     ~~~~~~~~~~~~~~~~~~~~~~~~
    │
    └─ test/authorize_test.exs:51: Item."admins can update and delete any item"/3