Update phpstan and related dependencies to latest version

Problem/Motivation

Our dev dependency on phpstan and phpstan-drupal is not up to date.

mglaman/phpstan-drupal 1.1.9 1.1.16 Drupal extension and rules for PHPStan
phpstan/phpstan 1.4.0 1.6.7 PHPStan - PHP Static Analysis Tool

Updating those provides us with the latest improvements from both.
For the phpstan updates this might give us performance improvements. (which might help with #3259355: Always do a full phpstan analysis on DrupalCI)
https://phpstan.org/blog/phpstan-1-6-0-with-conditional-return-types

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet