Remove RequestOptions.setStripeVersionOverride

[richardm-stripe]

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cc @stripe/api-library-reviewers

Summary

This PR removes RequestOptions.setStripeVersionOverride. There was only one supported use case for setStripeVersionOverride, for EphemeralKey.create. Now, users should

EphemeralKeyCreateParams params = EphemeralKeyCreateParams.builder()
  .setStripeVersion("XXXX-YY-ZZ")
  .build();

to set the Stripe-Version header appropriately.

We introduce RequestOptions.unsafeSetStripeVersionOverride which is marked as internal only.
Because com.stripe.model.EphemeralKey is not in the com.stripe.net package, we can't actually change make this internal-only method private, but users should not use it.

Changelog

• Add support for StripeVersion as a required value on EphemeralKeyCreateParams.
• Add support for EphemeralKey.create without specifying RequestOptions.
• RequestOptions.setStripeVersionOverride is renamed to unsafeStripeVersionOverride to confront upgrading users with the fact that this is now internal-only and should not be used. The docs now indicate it is internal-only. It is also now a static method, to prevent it from polluting IDE autocomplete list.
• RequestOptions.getStripeVersionOverride is similarly changed to unsafeGetStripeVersionOverride.
• RequestOptions.clearStripeVersionOverride is removed entirely. There is no reason to use this.
• RequestOptions.toBuilder is now marked as deprecated, in favor of toBuilderFullCopy, which has clearer semantics.

[richardm-stripe]

Assigned you @pakrym-stripe as you are the "reporter" on our internal ticket for this ;)

[pakrym-stripe]

can we make this a static method so it doesn't show up in IDE code completion. And maybe name it unsafeSetStripeVersionOverride?

[pakrym-stripe]

LGTM overall. Wonder if we can hide _setStripeVersionOverride in a way that doesn't pollute code completion.

[pakrym-stripe]

One nit, let's hide the getVersionOverride