BE: Data migration to update all existing ItemConsumer records that do not have a pinned_version_prefix

Proposal

After #578589 (closed) has been deployed, any new ItemConsumers will have a pinned_version_prefix.

We should then deploy a data migration to update all existing ai_catalog_item_consumers records with null pinned_version_prefix values to be pinned to the latest version of the associated ai_catalog_items record instead. The latest version of the item can be determined by the version value of the most recently created ai_catalog_item_versions for that item.