[14.x] Remove unique index from items table

[driesvints]

I stumbled upon an edge case where Stripe created a new subscription item for the same price ID. This means Cashier attempts to insert this while the unique constraint is active. Although it's unlikely this happens often (we only saw this just a couple of times) we shouldn't rely on this index anymore.

[shatterproof]

Migration to manually implement this change:

public function up()
{
    Schema::table('subscription_items', function (Blueprint $table) {
        $table->dropUnique(['subscription_id', 'stripe_price']);
        $table->index(['subscription_id', 'stripe_price']);
    });
}

Note that if you upgraded from Cashier 12.x you may need to replace dropUnique(['subscription_id', 'stripe_price']) with dropUnique(['subscription_id', 'stripe_plan']) since the unique index could still be named with the old column.