Improve type-annotations for currently untyped code

[Kingdutch]

PHP 8.1 will allow us to add new types to our classes that weren't available yet in PHP 7.4.

We should use this opportunity of a new major version to improve static type coverage in our code.

[klausi]

This is being done in https://www.drupal.org/project/graphql/issues/3513159 in preparation for 5.0.