Update to current version of stripe/stripe-php

As long as the API doesn't change from _this_ module, we should be able to handle a lot of this via composer dependencies. But show some code and it will be easier to decide if a new major version is needed or not.

Has anyone made any progress on this? I am seeing some console/js issues with payment_intents and haven't fully debugged, but wondering if this is related to the age of the 7x version and v1 API.