Fix deprecation around licencing in toml

[Cadair]

Describe the bug

/home/stuart/Git/ndcube/.tox/.pkg/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 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.
        ********************************************************************************

This probably needs to be done upstream

To Reproduce

import sunpy

sunpy.map.Map(...)
etc

Screenshots

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System Details

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Installation method

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