chore: adjust license metadata in pyproject.toml

[mxschmitt]

This fixes:

C:\Users\runneradmin\AppData\Local\Temp\build-env-sd_so16a\lib\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 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.
        ********************************************************************************
!!
  corresp(dist, value, root_dir)
C:\Users\runneradmin\AppData\Local\Temp\build-env-sd_so16a\lib\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 :: Apache Software License
        See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
        ********************************************************************************
!!
  dist._finalize_license_expression()
C:\Users\runneradmin\AppData\Local\Temp\build-env-sd_so16a\lib\site-packages\setuptools\dist.py:759: SetuptoolsDeprecationWarning: License classifiers are deprecated.
!!
        ********************************************************************************
        Please consider removing the following classifiers in favor of a SPDX license expression:
        License :: OSI Approved :: Apache Software License
        See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
        ********************************************************************************
!!