Retry for all error codes >= 500

[ramya-stripe]

This PR updates the retry behavior in the Ruby SDK to be the same as rest of our languages in terms of when to retry for errors with code >=500

From @richardm-stripe who lead this investigation previously

The additional restrictions in stripe-go/stripe-ruby (don't bother retrying POST status=500) is a sensible optimization but the logic also misses some statuses (503/504/502) that possibly could succeed on retry. It's better for cases more granular than >=500 to be captured in the logic that produces x-stripe-should-retry rather than to be hardcoded/maintained across 7+ official server SDKs, Terminal SDKs, direct integrations, etc.

Related Jira: https://jira.corp.stripe.com/browse/DEVSDK-1711

Changelog

• When no x-stripe-should-retry header is set in the response, the library now retries all requests with status >= 500, not just non-POST methods.
• Remove method parameter from StripeClient.should_retry?