v4 API: Support Adding Subscribers to Legacy Forms

[n7studios]

Summary

Uses this PR and some logic to call the applicable v4 API method, depending on whether the Plugin needs to add the subscriber to a Form or Legacy Form.

Testing

• Reinstated Legacy Form ID in tests that were changed in this previous PR, to confirm subscribers are added to Legacy Forms.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)