/**
 * Use this function to open the manage subscriptions page.

 * - Parameter completion: A completion block that is called when the modal is closed.
 * If it was not successful, there will be an `Error`.

 * If the manage subscriptions page can't be opened, the ``CustomerInfo/managementURL`` in
 * the ``CustomerInfo`` will be opened. If ``CustomerInfo/managementURL`` is not available,
 * the App Store's subscription management section will be opened.
 *
 * The `completion` block will be called when the modal is opened, not when it's actually closed.
 * This is because of an undocumented change in StoreKit's behavior between iOS 15.0 and 15.2,
 * where 15.0 would return when the modal was closed,
 * and 15.2 returns when the modal is opened.
 */