[14.x] Fix wrong foreignKey on SubscriptionItem

[zofe]

This pull request fixes an error in the relationship between items and subscription when using custom models

i.e. using

        Cashier::useSubscriptionModel(StripeSubscription::class);
        Cashier::useSubscriptionItemModel(StripeSubscriptionItem::class);

With these I think "licit" migrations

        Schema::create('stripe_subscriptions', function (Blueprint $table) {
            $table->id();
            ....
        });
        Schema::create('stripe_subscription_items', function (Blueprint $table) {
            $table->id();
            $table->foreignId('stripe_subscription_id');
            ...
        });

current codebase works in finding the items from the subscription, but not in finding the subscription from the items.
so $item->subscription will never works without this fix.

Of course, it can be solved by overriding the relation in our custom SubscriptionItemModel..
but I think that is a correct pr.

[driesvints]

Lgtm 👍