Enforce return types to non-inheritable navigation methods

Problem/Motivation

Once ✨ Enforce return types in all new methods and functions RTBC has been merged, a bunch of PHPStan ignore rules related to Navigation module were added to .phpstan-baseline.php.

Steps to reproduce

N/A

Proposed resolution

Review the Navigation module related ignore rules in .phpstan-baseline.php and update the Navigation module code accordingly to get rid of those silenced errors.

Changes should be applied only in methods that are not suitable to be reused by other modules in order to avoid BC breaks.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet