Set Api Version in Stripe Gateway

[mnelson4]

Seth forwarded me an email from a Stripe rep (possibly a Bot, but let's not discriminate) suggesting we should use setApiVersion in our integration. They didn't really specify which version, but it's probably a good idea. I created a ticket for EE4, but it applies to EE3 too.
If you don't set it, you can set the version in the Stripe dashboard. So far, I haven't seen any changes in the last few years that would affect us (I think), but it's possible they may introduce a backward-incompatible change that does affect us, then new users may default to that version before we have a chance to update our integration (which would be lots of fun to debug I'm sure.) More details on the EE4 Stripe ticket.