Update environment: pin `iris>=3.11`, unpin `cartopy` and allow for `numpy >=2`

[valeriupredoi]

Description

• We are now using iris=3.11 which is what we need to be able to unpin cartopy, see @schlunma 's comment https://github.com/ESMValGroup/ESMValTool/issues/3769#issuecomment-2456938359
• to not risk getting cartopy==0.24 with an older iris eg iris==3.10, and hence get the bug we pinned cartopy for in the first place, we should pin iris>=3.11
• iris==3.11 works well with numpy>=2 so we remove the upper pin in our own environment; note, however, that numpy2 is not yet available to us, due to ncl (that holds esmpy from going past 8.6, and makes us keep that silly importlib_metadata<8 pin, see https://github.com/ESMValGroup/ESMValTool/issues/3699)

Closes #3769 Contributes to https://github.com/ESMValGroup/ESMValTool/issues/3812

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[valeriupredoi]

cheers for the review, Manu :beer: