R1:a110aa471e98

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…