Default units of "1" or "unknown"

[hdyson]

✨ Feature Request

Iris 3 changed the default unit for a cube from "1" to "unknown". As part of the work to upgrade ANTS to use iris 3, a couple of functions were altered to preserve the iris 2 behaviour: here and here.

Requirements

We should:

1. Confirm the iris 3 behaviour is consistent with CF conventions (it most likely is, but it's worth checking).
2. Remove those workarounds to instead use the iris 3 behaviour.
3. Evaluate the impact. For F03 ancillary files, there should be no change (the unit information is defined by the STASH code rather than the units attribute), so testing should focus on UKCA netCDFs and UG-ANTS/LFRic.

Related issue: https://github.com/MetOffice/ancillary-file-science/issues/33