Update iris netcdf saving - causing warnings in standard out

[lewisblake]

Describe the bug
This warning is all over standard out:

warnings.warn(
/modules/rhel8/user-apps/fou-kl/aerotools/pya-v2024.09/lib/python3.11/site-packages/iris/fileformats/netcdf/saver.py:2664: IrisDeprecation: Saving to netcdf with legacy-style attribute handling for backwards compatibility.
This mode is deprecated since Iris 3.8, and will eventually be removed.
Please consider enabling the new split-attributes handling mode, by setting 'iris.FUTURE.save_split_attrs = True'.

• Pyaerocom version: 0.23.dev0
• Computing platform: all
• Configuration file (if applicable): NA
• Error message (if applicable): NA

To Reproduce
Steps to reproduce the behavior:

1. Run a config, e.g., obs only

Expected behavior
Not for this warning to appear