Fix PHPStan L1 errors "#foo callback 'bar' at key 'n' is not callable."

Problem/Motivation

Amongst the current suppressions found in the PHPStan level 1 baseline is: #foo callback 'bar' at key 'n' is not callable..

This issue exists to fix all of those.

Steps to reproduce

- Run PHPStan on level 1 and see the above issue amongst all others.

Proposed resolution

- Solve all of the reported issues for the above mentioned. As a matter fact, both occurrences are purposely invalid callbacks, so we need to @phpstan-ignore-next-line them.
- Run PHPStan on level 1 and don't see the above issue any more.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet