Finish to setup PHPStan

[marien-probesys]

Problem

We've configured PHPStan, but it reports a bunch of errors (ignored thanks to the baseline). These are quite important errors though.

Describe the solution you'd like

Remove the phpstan-baseline.neon file and fix the issues. Also, we must consider to increase the PHPStan level one by one, up to at least level 7.

Describe alternatives you've considered

If there are too many errors at the upper levels, stop if it takes too long to fix the problems.

Additional context

N/A