Update ConvertKit_API_Traits class from PHP SDK

[n7studios]

Summary

Uses the latest version of the PHP SDK trait file

Moves the convert_script_type method to the ConvertKit_API_V4 class, as it's not used in the new PHP SDK trait file. This ensures future updates to the PHP SDK trait can be directly copied to these libraries.

Testing

Existing tests pass.

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)