[GRASS-git] [OSGeo/grass] b5cf23: CITATION.cff: make license entry SPDX conform (#3115)

[Markus Neteler]

  Branch: refs/heads/releasebranch_8_3
  Home:   https://github.com/OSGeo/grass
  Commit: b5cf23b676d49d7b42c88e3b8dbc836d7a77989f
      https://github.com/OSGeo/grass/commit/b5cf23b676d49d7b42c88e3b8dbc836d7a77989f
  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