Make Iris compatible with Matplotlib 3.11

[trexfeathers]

#7166 introduces this pin:

iris/requirements/py314.yml

Line 20 in 5d7ca14

- matplotlib-base >=3.5, !=3.9.1, <3.11

We think this was to enable updating of the dependencies for Towncrier, and was intended to be temporary, so this issue captures the intent to make Iris compatible with Matplotlib v3.11. This will involve some image test updates.

[rcomer]

While at it, I suggest to also switch to using the testing label, as is done in Cartopy
https://github.com/SciTools/cartopy/blob/caee026cb966d88d9b24bc058c3225e8b9457742/environment.yml#L21-L22

This label has Freetype pinned to a specific version, consistent with the wheels available in PyPI. The Freetype pin means that text changes appear significantly less often than when using the main Conda Forge build of Matplotlib, so baseline images will need updating less frequently in future. I’m not sure if this is documented anywhere, but it was explained to me here SciTools/cartopy#2666 (comment).

[rcomer]

A downside of my above suggestion is that image tests would fail if/when you run them in a Met Office user environment that has the standard Matplotlib installed.