Add support for phpstan 1.12

[hostep]

Hiya!

Would be nice if the module can be released with support for phpstan 1.12, see:

• https://github.com/phpstan/phpstan/releases
• https://phpstan.org/blog/phpstan-1-12-road-to-phpstan-2-0

It sounds like this is the last minor version release line before they'll release 2.0

Maybe we can consider switching again to ^1.11.1 constraint instead of ~1.11.1 for phpstan, as it's supposed to be backwards compatible?

Already tried with phpstan version 1.12.0 on 4 small modules and 1 semi larger project, no issues found.

[shochdoerfer]

Thanks for testing :)

Will try to merge #326 soonish.

[shochdoerfer]

@hostep here you go https://github.com/bitExpert/phpstan-magento/releases/tag/v0.32.0