feat: update PHPStan lvl to 5 - #91

Your requirements could not be resolved to an installable set of packages. Problem 1 - Root composer.json requires phpunit/phpunit >=9.0 <9.6, found phpunit/phpunit[9.0.0, ..., 9.5.28] but these were not loaded, because they are affected by security advisories ("PKSA-z3gr-8qht-p93v"). Go to https://packagist.org/security-advisories/ to find advisory details. To ignore the advisories, add them to the audit "ignore" config. To turn the feature off entirely, you can set "block-insecure" to false in your "audit" config.