Fix spacing above CTA button in email templates

[officialasishkumar]

Summary

Addresses issue:

• Incorrect spacing above CTA button (16px instead of 20px) in subscription confirmation and error email templates #12344

Relevant technical choices

The subscription-confirmation and error-email templates had a fixed 16px bottom margin on all body paragraphs, resulting in only 16px spacing above the CTA button. The invitation-email template already handled this correctly by applying 20px to the last paragraph.

This change applies the same conditional pattern from the invitation template to both affected templates:

• Track the loop index via foreach ( $body as $index => $paragraph )
• Use count( $body ) - 1 === $index ? '20px' : '16px' for the last paragraph's bottom margin

This ensures consistent 20px spacing above the CTA button across all three email templates while keeping the 16px spacing between intermediate paragraphs.

PR Author Checklist

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 5.2 and PHP 7.4.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have added a QA Brief on the issue linked above.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).

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Code Reviewer Checklist

• Run the code.
• Ensure the acceptance criteria are satisfied.
• Reassess the implementation with the IB.
• Ensure no unrelated changes are included.
• Ensure CI checks pass.
• Check Storybook where applicable.
• Ensure there is a QA Brief.
• Ensure there are no unexpected significant changes to file sizes.

Merge Reviewer Checklist

• Ensure the PR has the correct target branch.
• Double-check that the PR is okay to be merged.
• Ensure the corresponding issue has a ZenHub release assigned.
• Add a changelog message to the issue.