fix license string

[s-sajid-ali]

To prevent this error when including this package as a dependency for the coldfront plugin:

  × Failed to build `slurm-python-sdk @ git+https://github.com/coldfront/slurm-python-sdk.git`
  ├─▶ The build backend returned an error
  ╰─▶ Call to `setuptools.build_meta.build_wheel` failed (exit status: 1)

      [stdout]
      configuration error: `project.license` must be valid exactly by one definition (0 matches found):

          - {type: string, format: 'SPDX'}
          - type: table
            keys:
              'file': {type: string}
            required: ['file']
          - type: table
            keys:
              'text': {type: string}
            required: ['text']

      DESCRIPTION:
          `Project license <https://peps.python.org/pep-0621/#license>`_.

      GIVEN VALUE:
          "Apache 2.0"

      OFFENDING RULE: 'oneOf'
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