[Sync EN] ReturnTypeWillChange: Add warning about future strict return type (#5613)

[github-actions]

Commit: https://github.com/php/doc-en/commit/0019a7e201442447fd746c2852d28ba839ed15ae
PR: https://github.com/php/doc-en/pull/5613

Zu aktualisierende DE-Dateien

• language/predefined/attributes/returntypewillchange.xml