Inconsistent variable naming for 'z' and 'zs' in data repository and storage bucket.

[dabhicusp]

As per ECMWF's official website documentation, the variable name should be referenced as z and not zs. This is indicated in the reference link: (https://codes.ecmwf.int/grib/param-db/?id=129). However, it's observed that both in the repository (https://github.com/google-research/arco-era5/blob/main/raw/era5_ml_zs.cfg) and within the storage bucket(gs://gcp-public-data-arco-era5/raw/ERA5GRIB/HRES/Month/**) where the data is stored, the variable name is currently set as zs. To ensure consistency, an update is required in this regard.