[CI] Run PHPStan job in the GitLab project directory instead of /build

Problem/Motivation

Follow up of 📌 [CI] Run PHPStan job on PHP 8.4 Active .

PHPStan now uses a baseline in PHP format that stores path information in a relative path, so we no longer need to run in the /build staging directory.

Proposed resolution

Change the CI job so that it PHPStan can run from the GitLab project path.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet