Make use of PHPStan 1.8 collector to add unused rule

[TomasVotruba]

We had to drop this feature when moving to parallel, because PHPStan had only single-file scope. Now it's back so we can return the rules with whole-project-scope :)

• See application update: https://github.com/phpstan/phpstan-src/commit/8676d4cc07f99f090f1b27b170ad8a723d55a8ea
• Docs: https://phpstan.org/developing-extensions/collectors