Test against minimum versions for dependencies

[VeckoTheGecko]

From #2205

We don't have much control over the versions of dependencies installed in this environment, with it just defaulting to the latest versions (we don't test against minimum supported versions of other core packages (xarray, zarr, uxarray). This means that our lower pinning in our environment.yml file as well as our feedstock is likely out of date).

It would be good to fix the lower pinning of our dependencies and test against this in CI. Note that we follow SPEC0 in terms of the packages that we support.

This would look like the following in pixi.toml, with some comments re. implementation (no need to have those comments in the final thing):

[environments]
test-minimal = { features = ["test", "minimal"], solve-group = "test" }

[dependencies]
# versions here will be manually walked up according to SPEC0 when they're noticed to be out of date
# or will be bumped to make sure that we have new features only available in newer releases.
# Once these are bumped, the minimum should also be bumped (but that will be clear anyway - 
# since the pixi lock process will informatively fail anyway)
numpy = ">=1.9.1"
xarray = ">=0.10.8" 

[feature.minimum.dependencies]
# here we pin to the lowest versions for our default dependencies
python = "==3.11"
numpy = "==1.9.1"
xarray = "==0.10.8" 

Also need to update CI matrix with the test minimal dep

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If you want to dip your feet into Pixi @andrew-s28 , this would be a good issue ;)

[andrew-s28]

I think this is closed by #2301 @VeckoTheGecko