PHP 8.1 + Symfony 5.4 new requirements

[williarin]

This PR introduces new requirements for the future versions of the CMS.

• PHP 8.1 as minimum version
• Symfony 5.4 as minimum version
• Themes must now use attribute #[Theme] instead of annotation
• Shortcodes must now use attribute #[Shortcode] instead of annotation

Also replaced PHPCodeSniffer by EasyCodingStandards.