update license specifier

[orbeckst]

Expected behavior

Project metadata for license is up to date and uses a SPDX License identifier for GNU General Public License v2 or later (GPLv2+), i.e., replace with GPL-2.0-or-later

Actual behavior

Please use a simple string containing a SPDX expression for project.license

Output from GitHub action showing deprecated identfiers

/home/runner/miniconda3/envs/test/lib/python3.14/site-packages/setuptools/config/_apply_pyprojecttoml.py:82: SetuptoolsDeprecationWarning: `project.license` as a TOML table is deprecated
!!
    ********************************************************************************
    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 2027-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.
    ********************************************************************************

!!

corresp(dist, value, root_dir)

/home/runner/miniconda3/envs/test/lib/python3.14/site-packages/setuptools/config/_apply_pyprojecttoml.py:82: SetuptoolsWarning: install_requires overwritten in pyproject.toml (dependencies)

corresp(dist, value, root_dir)

/home/runner/miniconda3/envs/test/lib/python3.14/site-packages/setuptools/config/_apply_pyprojecttoml.py:82: SetuptoolsWarning: extras_require overwritten in pyproject.toml (optional-dependencies)

corresp(dist, value, root_dir)

/home/runner/miniconda3/envs/test/lib/python3.14/site-packages/setuptools/config/_apply_pyprojecttoml.py:61: SetuptoolsDeprecationWarning: License classifiers are deprecated.

!!
    ********************************************************************************
    Please consider removing the following classifiers in favor of a SPDX license expression:

    License :: OSI Approved :: GNU General Public License v2 or later (GPLv2+)

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

!!

dist._finalize_license_expression()

/home/runner/miniconda3/envs/test/lib/python3.14/site-packages/setuptools/dist.py:765: SetuptoolsDeprecationWarning: License classifiers are deprecated.

!!
    ********************************************************************************
    Please consider removing the following classifiers in favor of a SPDX license expression:

    License :: OSI Approved :: GNU General Public License v2 or later (GPLv2+)

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

Code to reproduce the behavior

See https://github.com/MDAnalysis/mdaencore/actions/runs/24868013657/job/72808287102 "Build and install package"