CI updates & PHP 8.3

This updates and cleans up the CI tests, main changes:

• Run tests on PHP 8.3 too
• Actually run tests on the selected PHP version, the version matrix was basically unused
• Run tests on all supported PHP versions except PHP 7.1 which is not supported by PHPStan
• Fix #192 thanks to running tests on the proper PHP versions
• Documents supported PHP versions
• Drops Travis CI badges, files as Travis is not used anymore and provides outdated info