fix(recaptcha): also verify v2 on checkout

[chickenn00dle]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Closes https://app.asana.com/0/1200550061930446/1209016713141551/f

This PR makes sure we are also verifying v2 recaptcha response on checkout requests.

How to test the changes in this Pull Request:

1. Enable recaptcha v2
2. As a reader go through checkout
3. There should be no change in behavior, but you can add some logging here to confirm the recaptcha verification is happening in the backend:

   newspack-plugin/includes/class-recaptcha.php

   Line 504 in 731535d

   $check = self::verify_captcha();

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?