Update pyproject.toml License file specification

[ReeceHumphreys]

Describe the bug
Specifying the license file as a table is deprecated:

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.
          ********************************************************************************

However, Python 3.8 does not support the latest way of specifying it which is like this:

[project]
license = "ISC"
license-files = ["LICENSE"]

As such, when Python 3.8 support is dropped we should update the pyproject.toml to use the new way of specifying the license file.