Use latest Iris (3.13) with correct derived coordinates loading

[bouweandela]

Description

Since Iris 3.13, loading derived coordinates should work better: https://scitools-iris.readthedocs.io/en/stable/whatsnew/index.html#v3-13-07-aug-2025

SciTools/iris#3678

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[valeriupredoi]

there is some oddity going on in the test - I'm not sure why there is a newline \n element in a CubeList

[schlunma]

Hello, this pull request has been marked with the v2.14.0 milestone. The release of version 2.14.0 is currently scheduled for February 2026. To get this into the new release, it would be great to get this merged by the end of January.

If you won't be able to finish this in time, don't worry - just unassign the milestone v2.14.0. If you need any support, ping myself (@schlunma; the release manager for v2.14.0) or the @ESMValGroup/technical-lead-development-team. Please note that I won't be available until the beginning of February, though.