Suppress PHP 8.1 warnings "Return type mixed is not covariant with tentative return type mixed of method Iterator::key()"

Summary:
PHP 8.1 requires classes which implement Iterator to have all Iterator methods declared with appropriate return types or annotated with #[\ReturnTypeWillChange]. In my (limited) understanding we cannot declare return types while remaining backward-compatible, thus annotate.

Closes T15835

Test Plan: Run PHPStan static code analyzer, see no warnings anymore.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15835

Differential Revision: https://we.phorge.it/D25711