Bump PHPStan to level 2.

[mglaman]

Many deprecations are missed due to PHPStan not detecting correct methods because code is not typed appropriately. Solving "undefined methods" will fix detection of deprecated methods. To do so, we'd need to bump to level 2.

Examples of why we need to:

• https://www.drupal.org/project/upgrade_status/issues/3131063
• Node::getRevisionAuthor deprecation not detected phpstan-drupal#210

[jkribeiro]

I just think it should have been done in a Major release, but that's ok.