Importers: Migrate Kit Legacy Forms

[n7studios]

Summary

As the Kit API evolves, existing API versions will be deprecated - which at some point would include Kit's legacy forms.

This PR adds an option in Settings > Kit > Tools to replace Kit Legacy forms with Kit non-legacy forms across shortcodes and blocks:

Future PR's to follow covering:

• a notice in the Plugin settings screens to migrate away from legacy forms,
• notices in Post, Category and Plugin setting screens where legacy forms are specified,
• removing legacy forms from dropdown selectors
• removing calls to legacy forms
• removing support for v1, v2 and v3 APIs

The idea is users are encouraged to migrate to non-legacy forms (given some refuse to move away from them), with the plugin being more and more opinionated over time as deprecation deadlines appear.

Testing

• See Importers: Migrate Kit Legacy Forms: Tests #1128 for tests

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)

[github-actions]

WordPress Playground

🚀 Your PR has been built and is ready for testing in WordPress Playground!

Click here to test your changes in WordPress Playground