This fixes the "Deprecated: Return type of `WP_Block_List::[METHODNAME]()` should be compatible with `ArrayAccess::[METHODNAME](): type`" warnings on PHP 8.1.
PHP native interfaces now have declared return types and methods in classes implementing these interfaces need to either have the return type declared (in a covariant compatible manner with the PHP native interface method declaration), or need to silence the deprecation warning using the `#[ReturnTypeWillChange]` attribute.
Follow-up to [51517], [51529], [51530], [51531].
Props jrf.
See #53635.
git-svn-id: https://develop.svn.wordpress.org/trunk@51532 602fd350-edb4-49c9-b593-d223f7449a82