Update `pyproject.toml` license info according to PEP 639

[flferretti]

This pull request updates the licensing information in the pyproject.toml file to align with the BSD-3-Clause license standard and removes an outdated classifier entry.

Licensing updates:

• Updated the license field to specify "BSD-3-Clause" and added a license-files field to include the LICENSE file, replacing the previous license.file entry. (pyproject.toml, pyproject.tomlL14-R15)

Classifier adjustments:

• Removed the outdated classifier "License :: OSI Approved :: BSD License" from the classifiers list, as it is no longer needed with the updated license field. (pyproject.toml, pyproject.tomlL32)#license and https://peps.python.org/pep-0639/

---

📚 Documentation preview 📚: https://jaxsim--415.org.readthedocs.build//415/