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.
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