Increase PHPstan to 6 , then 7, then 8 , then 9 

### Feature Request



|    Q        |   A
|------------ | ------
| New Feature | yes
| RFC         | no
| BC Break    | no

#### Summary

Try to implement PHPstan at level 6 
to estimate the effort required 

make a PR with level 6

then try to go to level 7 

then try to go to level 8

then try to go to level 9