Bump stripe/stripe-php to v16

[karkowg]

Bump stripe/stripe-php to latest version (16.2). Some Stripe features like invoice preview were not available prior to 15.x.

[karkowg]

I have a couple of tests failing locally that I have yet to find the time to investigate.

There were 2 errors:

1) Laravel\Cashier\Tests\Feature\CustomerTest::test_customers_can_generate_a_billing_portal_url
Stripe\Exception\InvalidRequestException: You can’t create a portal session in test mode until you save your customer portal settings in test mode at https://dashboard.stripe.com/test/settings/billing/portal.

/cashier-stripe/vendor/stripe/stripe-php/lib/Exception/ApiErrorException.php:38
/cashier-stripe/vendor/stripe/stripe-php/lib/Exception/InvalidRequestException.php:35
/cashier-stripe/vendor/stripe/stripe-php/lib/ApiRequestor.php:230
/cashier-stripe/vendor/stripe/stripe-php/lib/ApiRequestor.php:191
/cashier-stripe/vendor/stripe/stripe-php/lib/ApiRequestor.php:637
/cashier-stripe/vendor/stripe/stripe-php/lib/ApiRequestor.php:136
/cashier-stripe/vendor/stripe/stripe-php/lib/BaseStripeClient.php:184
/cashier-stripe/vendor/stripe/stripe-php/lib/Service/AbstractService.php:75
/cashier-stripe/vendor/stripe/stripe-php/lib/Service/BillingPortal/SessionService.php:25
/cashier-stripe/src/Concerns/ManagesCustomer.php:465
/cashier-stripe/tests/Feature/CustomerTest.php:96

2) Laravel\Cashier\Tests\Feature\CustomerTest::test_customers_can_be_redirected_to_their_billing_portal
Stripe\Exception\InvalidRequestException: You can’t create a portal session in test mode until you save your customer portal settings in test mode at https://dashboard.stripe.com/test/settings/billing/portal.

/cashier-stripe/vendor/stripe/stripe-php/lib/Exception/ApiErrorException.php:38
/cashier-stripe/vendor/stripe/stripe-php/lib/Exception/InvalidRequestException.php:35
/cashier-stripe/vendor/stripe/stripe-php/lib/ApiRequestor.php:230
/cashier-stripe/vendor/stripe/stripe-php/lib/ApiRequestor.php:191
/cashier-stripe/vendor/stripe/stripe-php/lib/ApiRequestor.php:637
/cashier-stripe/vendor/stripe/stripe-php/lib/ApiRequestor.php:136
/cashier-stripe/vendor/stripe/stripe-php/lib/BaseStripeClient.php:184
/cashier-stripe/vendor/stripe/stripe-php/lib/Service/AbstractService.php:75
/cashier-stripe/vendor/stripe/stripe-php/lib/Service/BillingPortal/SessionService.php:25
/cashier-stripe/src/Concerns/ManagesCustomer.php:465
/cashier-stripe/src/Concerns/ManagesCustomer.php:481
/cashier-stripe/tests/Feature/CustomerTest.php:106

[karkowg]

@driesvints looks like I was just missing enabling a customer portal link on my Stripe account. All tests are passing now. Please let me know if I've missed anything else.

[taylorotwell]

Drafting pending @driesvints review