Do not use deprecated project.license TOML table

[mattwthompson]

See https://github.com/conda-forge/openff-units-feedstock/actions/runs/30559589434/job/90928851833#step:4:826

 │ │   $PREFIX/lib/python3.12/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)

This shows up in many places and I am to roll this out everywhere. The changes should be identical

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.37%. Comparing base (d745081) to head (9fa49fc).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #190   +/-   ##
=======================================
  Coverage   91.37%   91.37%           
=======================================
  Files           6        6           
  Lines         232      232           
=======================================
  Hits          212      212           
  Misses         20       20           

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[j-wags]

Great catch, thanks @mattwthompson. You'd mentioned being willing to roll this change out to all our repos - Please do, and consider this review as blanket approval to hero-merge the other PRs without review.