Support for PHP 8.1

• Support for PHP 8.1 (#840 by @alexander-schranz)
• Replaced Travis with Github Actions (#840 by @alexander-schranz, @romainneutron)
• Dropped Silex support (#815 by @franmomu)
• [BC] Replaced Doctrine Cache with Symfony Cache (#824 by @ekyna, #816 by @pascalbaljet)
• Dropped support for PHP 5.3 and 5.4 (#816 by @pascalbaljet)