Stripe sdk version support

[lolmx]

Issue: breaking changes over major sdk releases

As stated in this and this comments through #4, Stripe v7 introduced two main BC:

• __toArray() became toArray()
• all exceptions have been renamed

We need to support both versions as far as possible.
At the moment i see two ways to supports both, we can add a new config parameter to ask for used sdk version which i'm not fan or add checks with class_exists and method_exists.
I need more inputs to see how we can properly supports both versions.

This also raise the question about supporting older versions ^3|^4|^5. Are they still needed? Anyone in need to use Strong Customer Authentication already must use Stripe ^6.9 to have access to newer api (PaymentIntent & SetupIntent for js gateway).
New Checkout Server Flow (#6) also requires new apis available in ^6.

Wouldn't be beneficial to drop support for ^3|^4|^5? And also maybe for versions >=6.0 <6.9?

[diimpp]

Dropping Legacy Checkout seems to be a right solution, if we will maintain a separate fork from Payum (Or new major version payum/stripe:2.0). Otherwise we will need to accommodate old sdk and old gateway as well as new ones. Legacy Charge API is still functional and for example USA is exempt from SCA requirements.