Use WordPress Libraries 2.6.0

[n7studios]

Summary

Uses the Kit WordPress Libraries 2.6.0, which have been updated to include the PHP SDK 2.6.0 traits.

This also matches the version number so that it's clearer which PHP SDK version is being used.

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)

[noelherrick]

I don't quite understand why we would merge an in-development version into main?

[n7studios]

I don't quite understand why we would merge an in-development version into main?

This PR will be updated to use the published 2.6.0 libraries. It's created using the branch for Kit/convertkit-wordpress-libraries#127 (which is a large PR) to demonstrate that it doesn't introduce any breaking changes.