Migrate `project.license` to SPDX Expression

[vytas7]

setuptools started nagging us with a slew of new warnings wrt project.license in pyproject.toml:

project.license as a TOML table is deprecated.

'tool.setuptools.license-files' is deprecated in favor of 'project.license-files' (available on setuptools>=77.0.0).

License classifiers are deprecated.

Let's update our licensing metadata according to the new recommendations:

• https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license
• https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license-files

[vytas7]

To reproduce, see, e.g., this CI gate: https://.com/falconry/falcon/actions/runs/14732142911/job/41348822392.