Integrate Psalm 6

[settermjd]

Feature Request

Q	A
New Feature	yes
RFC	no
BC Break	no

Summary

[tyrsson]

So with the release of psalm 6 the question is do we still want to go with phpstan or with psalm? @settermjd @simon-mundy @dragomano @halldorr

[tyrsson]

Decision has been made in favor of Psalm 6