Switch the license field to PEP 639 format once we can

[deltragon]

As of #788, we are using the deprecated PEP 621 format in order to support Ubuntu 24.04.
The newer PEP 639 format is only supported with setuptools >= 77, Ubuntu 24.04 has version 68.
Since we need an Ubuntu LTS to build .deb files on Github Actions, we can likely switch as soon as 26.04 is available there.
I am just opening this issue to track this, so I don't have to search for all this again next time I stumble over the warning.
It does seem that the deprecation period will get extended for another year, allowing us a good amount of time to switch: github.com/pypa/setuptools/pull/5115

(Note that this only affects whether we can build .deb files on Ubuntu 24.04 with the default packages. Using a virtualenv there should make it possible to build wheels after this switch still, and running the wheels is unchanged. I just don't see a way to make debuild use a virtualenv, and don't think there is one.)

This is the warning on setuptools >= 77:

/tmp/build-env-l2aslku1/lib/python3.13/site-packages/setuptools/config/_apply_pyprojecttoml.py:82: SetuptoolsDeprecationWarning: `project.license` as a TOML table is deprecated
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Please use a simple string containing a SPDX expression for `project.license`. You can also use `project.license-files`. (Both options available on setuptools>=77.0.0).

By 2026-Feb-18, you need to update your project and remove deprecated calls
or your builds will no longer be supported.

See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
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