Gribapi v1.9.16 --> v1.12.1 compatibility fix for GribMessage

made grib_get kwarg type an arg for v1.12.1 compatibility

Turns out that gribapi for Python was subtly updated between v1.9.16 and v1.12.1, to the extent that:
```python
gribapi.grib_get(gribid, key, type=None)
```
is no longer valid as the keyword `type` is now only a positional argument.

This PR updates the one line in `iris.fileformats.grib._grib_message.GribMessage` to reflect that change in behaviour.