phpstan: step back to level 4

[sreichel]

Description (*)

Think we made good progess. If we achieve L2/3/4 without ignoring/baseline errors, it would be great. Setting checks to L5 was to optimistic ... I already spent "hours", but without any help it is just to much.

Questions or comments

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[tmotyl]

@sreichel In my opinion level 5 was ok to keep, (even with big baseline) because it prevents new code from introducing new errors.

[sreichel]

because it prevents new code from introducing new errors.

For this reason i've reverted that ignored errors. Level5 adds ~1000 errors to baseline.

Better l4 w/o ignoring something and fix existing ones. We still have +30 errors at level 0, that should be investigated.