Wagtail: Improper restriction handling on descendant collections in Documents and Images API

Summary

### Impact The Documents and Images [API V2](https://docs.wagtail.org/en/stable/advanced_topics/api/index.html) incorrectly listed items in descendants of private collections, which should inherit the view restrictions defined on their ancestors. A user with access to the API could see the filename and name of documents and images in these descendant collections. ### Patches Patched versions have been released as Wagtail 7.0.9, 7.3.4, 7.4.3 and 8.0rc2. ### Workarounds Site owners using Wagtail's API can avoid the vulnerability by adding [authentication](https://docs.wagtail.org/en/stable/advanced_topics/api/v2/configuration.html#authentication) to the Documents and Images APIs. ### Acknowledgements Many thanks to Ta Duc Thien for reporting this issue. ### For more information If you have any questions or comments about this advisory: * Visit Wagtail's [support channels](https://docs.wagtail.org/en/stable/support.html) * Email us at [[email protected]](mailto:[email protected]) (view our [security policy](https://github.com/wagtail/wagtail/security/policy) for more information).