Refresh climatology support

# ✨ Feature Request
Iris 2 featured improved support for climatologies. We should investigate what these additional features are and use them where appropriate, including any further improvements with iris 3.

This may mean retiring `ancil.fileformats.ancil.preprocessing.set_climatology_year` and refreshing appendix A in the documentation.

Start by investigating the additional climatology features in Iris 2 and any further improvements in Iris 3. Then inspect ancil.fileformats.ancil.preprocessing.set_climatology_year and appendix A in the documentation. Done means determining which features are appropriate for ANTS, deciding whether the function should be retired, and refreshing the appendix as needed.

Written by the indexing model from the issue text.