snapshots: hard_deletes not defined in dbt_project schema

As per the [dbt documentation](https://docs.getdbt.com/reference/resource-configs/hard-deletes), the `hard_deletes` option is available from dbt-core v1.9 (alongside the existing `invalidate_hard_deletes`).

This configuration option is not present in the `latest` dbt_project-latest.json schema.