Feature/PHP upgrade from 7.1 to 7.4-8.1

In light of the T-App PHP upgrade to version 8.1.
Link to epic: https://trilations.atlassian.net/browse/TAPP-5328

• Made sure this package supports PHP versions: 7.4, 8.0, and 8.1 (using PHPCS)
• Expanded the .gitignore file
• Updated PHPUnit to the latest version
• Filled the README with relevant info

Additionally added a .gitlab-ci.yml file, to run the tests and to check the PHP compatibility.