Fix switching between payment processors for anonymous sessions

This fixes the following issues:

1. Switching payment processors in anonymous sessions (user does not have permission for api3 call).
2. Switching payment processors on drupal webform (when there are multiple Stripe processors).
3. Switching payment processors on backend forms (when there are multiple Stripe processors).

• We now specify the API version when configuring Stripe on the client-side.
• We now set the language for Stripe at the Stripe.js level instead of the card element level.

Original issue !100 (closed) found by @capo