DX: maintenance (PHP-CS-Fixer#5)

* DX: update PHP and deps

* CI

* update CS fixer

* apply CS

* migrate to PHP 7.1

* allow PHPUnit Bridge ^6 too

Allow PHP 8 and PHPUnit 8+9 (PHP-CS-Fixer#4)

Allow Symfony 5 + fix using the Process class (PHP-CS-Fixer#3)