fix(recaptcha): validate on form submit, not button click

[dkoo]

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:

Moves the event handler to render the reCAPTCHA v2 challenge to form submit instead of submit button click. This allows us to validate the form interaction no matter how the form is submitted—whether via submit button click, "enter" keypress, or even programmatically via JS.

How to test the changes in this Pull Request:

Test all protected flows protected by reCAPTCHA v2 by both clicking on the submit button and by hitting the "enter" key from the form inputs:

• Auth modal: registration
• Auth modal: login
• Registration block
• Newsletter Subscription Form block
• Checkout as an anonymous reader
• Checkout as a logged-in reader

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?

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[dkoo]

useCapture is necessary to prevent the gateways' iframes from bubbling their listeners to submit the checkout form separately.

[dkoo]

Superseded by #3676.