Posts: Replace wordpress/posts with v4/posts endpoint

[n7studios]

Summary

Removes the overrides in ConvertKit_API_V4 for get_posts() and get_post() that would query the private wordpress/posts endpoint.

Calls to get_posts() and get_post() methods are now handled by the ConvertKit_API_Traits trait, which queries the v4/posts endpoint.

This mirrors the PHP SDK.

Testing

Removed the test overrides in APINoDataTest and APITest. Tests responsible for get_posts() and get_post() are now handled by the TestsTrait.

Again, this mirrors the PHP SDK.

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)