Blocks: Upgrade to apiVersion 3

[n7studios]

Summary

Upgrades the Kit Plugin blocks in the block editor to WordPress' apiVersion 3 when the WordPress version is 6.3 or higher:
https://developer.wordpress.org/block-editor/reference-guides/block-api/block-api-versions/#version-3-wordpress-6-3

This is recommended ahead of WordPress 7.0, which will iframe the block editor:
https://developer.wordpress.org/block-editor/reference-guides/block-api/block-api-versions/block-migration-for-iframe-editor-compatibility/

WordPress installations >= 5.6 and < 6.3 will continue to use apiVersion 2, to ensure backward compatibility.

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)