[Symfony 5] Symfony\Contracts\EventDispatcher\Event subclasses need return types

Problem/Motivation

In Symfony 5, symfony/event-dispatcher-contracts has been bumped from v1 to v2 and return types have been added to isPropagationStopped() and stopPropagation().

Steps to reproduce

See #3161889: [META] Symfony 6 compatibility

Proposed resolution

Add return type hints to Drupal\Core\File\Event\FileUploadSanitizeNameEvent as this is the only class in core that implements one of these methods.

Remaining tasks

User interface changes

API changes

Anything that subclass Drupal\Core\File\Event\FileUploadSanitizeNameEvent and further modifies the method will also need to add a typehint, but this seems very unlikely.

Data model changes

Release notes snippet