lock stripe api_version used

[loicginoux]

for solidus_stripe v5:

Stripe api is versionned and the gem does not clarify what version is used. it should either lock it or make it configurable as an ENV variable. Not sure what is the best solution but, at least by default it should make it clear and use the last version available: https://stripe.com/docs/api/versioning It also ensure that the current documentation correspond to the api the gem is using

Stripe.api_version = '2022-11-15'