DX: update PHPStan

[kubawerlos]

I wonder if we should drop a caret from composer.json to avoid such surprises as the needed update is related to new PHPStan version.

// cc @keradus @SpacePossum @julienfalque

[coveralls]

Coverage remained the same at 91.572% when pulling 8c7af2a on kubawerlos:update-phpstan into bddb80c on FriendsOfPHP:2.19.

[julienfalque]

Yep, we should drop it and use an exact version (or commit compose.lock) so we don't get new errors randomly reported as new PHPStan versions are released. I would even apply this reasoning to all the tools we use (when possible).

[kubawerlos]

Let's go with PHPStan for now as it's the only one making troubles.

[keradus]

Thank you @kubawerlos.