[Symfony 6] Add typehints to Drupal\Core\Validation\DrupalTranslator

Problem/Motivation

From #3197482: Update Drupal 10 to depend on Symfony 5.4 (as a stepping stone to Symfony 6, for deprecation checking support)

Method "Symfony\Contracts\Translation\TranslatorInterface::trans()" might add "string" as a native return type declaration in the future. Do the same in implementation "Drupal\Core\Validation\DrupalTranslator" now to avoid errors or add an explicit @return annotation to suppress this message.

Method "Symfony\Contracts\Translation\TranslatorInterface::getLocale()" might add "string" as a native return type declaration in the future. Do the same in implementation "Drupal\Core\Validation\DrupalTranslator" now to avoid errors or add an explicit @return annotation to suppress this message.

Steps to reproduce

Proposed resolution

• Fix upstream to allow our return values - https://github.com/symfony/symfony/pull/44911
• Implement the typehints.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet