Allow testing of earliest/latest dependencies.

[seberg]

Mostly identical to gh-228, which allows the testing of oldest/latest dependencies. What oldest/latest dependencies means exactly has to be set by each project in their dependencies.yaml file for the test env.

I am interested in it, because I would like to start testing with NumPy 2, but don't like the idea of stopping testing with 1.x at the same time. But e.g. cudf testing with older pandas or cuGraph with older networkx would be good as well.

Modifications from draft PR:

• I renamed it to oldest, as suggested by Matthew.
• Noticed that the name is different between wheel and conda workflows, so modified both to include all matrix information.

(Marking as draft until I have linked PRs that show it working also for wheels, but Bradley did really all the work here, and it seems to work well.)

[seberg]

Hmmm, marking as non-draft either way. I don't think I can use this workflow reasonably in other PRs unless the branch lives directly in the shared workflows repo.

[bdice]

@seberg Correct — your fork won’t suffice for testing. I would recommend that you close this PR, request write access on this repo from ops, and push directly to my PR.