[GRASS-git] [OSGeo/grass] ead8e8: CITATION.cff: make license entry SPDX conform (#3115)

[Markus Neteler]

  Branch: refs/heads/main
  Home:   https://github.com/OSGeo/grass
  Commit: ead8e8dd3a5e5db4387e111c5f1ac31f1cd16016
      https://github.com/OSGeo/grass/commit/ead8e8dd3a5e5db4387e111c5f1ac31f1cd16016
  Author: Markus Neteler <neteler at osgeo.org>
  Date:   2023-08-08 (Tue, 08 Aug 2023)

  Changed paths:
    M CITATION.cff

  Log Message:
  -----------
  CITATION.cff: make license entry SPDX conform (#3115)

Use proper short code from https://spdx.org:

https://spdx.org/licenses/GPL-2.0-or-later.html