Rename authorV2 and enable author as nullable

As of 2023-01, the authorV2 field is being removed from the Storefront API. It was previously used to reference nullable author scenarios. As a result, the following changes are being made:

• authorV2 will now be deprecated.
• author will be used instead, and will now be a nullable field.

These changes will help improve consistency of the API and avoid confusion when using these fields.