Add PHP8 compatibility testing

[cadic]

Description of the Change

PHP compatibility tests include all versions starting from the oldest officially supported 5.3 to the current 8.1. Also updated WordPress coding standards linting to https://github.com/10up/phpcs-composer configuration.

Checklist:

• [x] I have read the CONTRIBUTING document.
• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my change.
• [x] All new and existing tests passed.

Changelog Entry

Changed - Automated testing code compatibility against PHP versions from 5.3 to 8.1

Credits

Props @cadic