Migrate license to new approach defined in PEP 639

[andy-maier]

setuptools has deprecated the current way of specifying the license in Python packages, which uses the following in a pyproject.toml file:

license = {text = "Apache License, Version 2.0"}
. . .
classifiers = [
    "License :: OSI Approved :: Apache Software License",
    . . .
]

The new approach uses this:

# License info in new PEP 639 format (since setuptools 77.0.3)
license = "Apache-2.0"
license-files = ["LICENSE"]

and requires upgrading the minimum version of setuptools to at least 77.0.3.

The new approach is expected to become mandatory in 2/2026.

This issue is to migrate to using the new approach.