Update project metadata to avoid using the deprecated way to declare the license.

Once https://github.com/scikit-learn/scikit-learn/pull/30746#pullrequestreview-2590397434 is merged, it should be possible to use the new standardized way to declare the licensing information in our `pyproject.toml` file. See:

https://peps.python.org/pep-0639/#deprecate-license-classifiers