Update to latest Stripe Checkout

[iandunn]

There's a new version of Checkout, and folks have started reporting problems w/ the old one, like payments failing because they don't support 3DS.

• https://stripe.com/docs/payments/checkout/migration#api-products
• https://stripe.com/docs/payments/accept-a-payment?integration=checkout
• https://stripe.com/docs/payments/checkout/migration#parameter-conversion

The places we need to update to use the new Checkout flow are:

• sponsorship payment
• camptix stripe addon

We could update WordCamp\Utilities\Stripe_Client to use the new Checkout flow (create a session ID and redirect to their stripe.com), or we could switch to using the official client instead.

Either way, it may be best to replace CampTix_Stripe_API_Client with the client chosen above, rather than updating and maintaining a 2nd custom client. If we stick with WordCamp\Utilities\Stripe_Client, then we'll need to port some of the functionality over, like refunds.