Add return types to UserCreationTrait

Problem/Motivation

As per #3461318: Enforce return types in all new methods and functions new classes that implement traits with methods that have a return type missing will fail phpstan.

Steps to reproduce

Proposed resolution

• In 11.x add proper return types. These are usually void as other types are often documented.
• In 10.4.x add a @return annotation, or a @phpstan-return annotation as that does not require a description, eg. for void return.
• Update phpstan baseline for hundreds of entries we no longer require.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet