Release version 1.21.1 - PHP 7.4 compatibility fixes

- Fix: PHP 7.4 errors due to strict function return types with union types
- Fix: Ensure bundled scripts & styles dependencies are properly loaded

Update asset versions and build for v1.20.6 release

- Update composer.json and package.json versions to 1.20.6
- Update plugin header version to 1.20.6
- Update readme.txt for WordPress.org release
- Finalize build assets for distribution

This completes the v1.20.6 hotfix release addressing WordPress 4.9
compatibility issues with has_blocks() and wp_date() functions.