Upgrade PHPStan to latest version (PrestaShop 1.7)

[Quetzacoalt91]

Since the first implementation of PHPStan in our development process, we did not increase the version because the docker image had changed. We were stuck on PHPStan - PHP Static Analysis Tool 0.12.89, a version released on 09/06/2021.

Keeping our tools up-to-date is important for the stability of our projects. However newer versions were using PHP 8 in their Docker image, preventing it to work with old version of PrestaShop. This PR add PHPStan as a composer dependency so we can control the version used with the lock file, and we updated the CI workflow so PrestaShop versions are manually downloaded and unzipped.

• [ ] Some errors will appear with the rules added. There are to way to fix this: Fix all the errors in the same PR, or add a baseline. I would prefer the first solution as it may fix some issues.

Replaces #1009