gnu: Rename json-modern-cxx to nlohmann-json.

* gnu/packages/cpp.scm (nlohmann-json): New variable.
(json-modern-cxx): Mark as deprecated alias.
(xtl, jsonnet)[inputs]: Use nlohmann-json.
* gnu/packages/build-tools.scm (bear)[inputs]: Likewise.
* gnu/packages/cybersecurity.scm (blacksmith)[inputs]: Likewise.
* gnu/packages/databases.scm (sqlitebrowser)[inputs]: Adjust comment.
* gnu/packages/games.scm (openrct2)[inputs]: Use nlohmann-json.
* gnu/packages/graphics.scm (f3d)[native-inputs]: Likewise.
* gnu/packages/hardware.scm (openrgb)[inputs]: Likewise.
[arguments]: Adjust accordingly.
* gnu/packages/image-processing.scm (paraview)[inputs]: Adjust comment.
* gnu/packages/irods.scm (irods, irods-client-icommands)[inputs]: Use
nlohmann-json.
* gnu/packages/jupyter.scm (xeus)[inputs]: Likewise.
* gnu/packages/messaging.scm (mtxclient, nheko)[inputs]: Likewise.
* gnu/packages/text-editors.scm (jucipp)[inputs]: Likewise.
* gnu/packages/video.scm (mktoolnix)[inputs]: Likewise.
* gnu/packages/xdisorg.scm (nwg-launchers)[inputs]: Likewise.

gnu/packages/image-processing.scm

@@ -1213,7 +1213,7 @@ in 3D or programmatically using ParaView’s batch processing capabilities.")
            glew
            gmsh
            hdf5
-           ;;json-modern-cxx              ;For ParFlow; build fails
+           ;;nlohmann-json                ;For ParFlow; build fails
            jsoncpp
            libjpeg-turbo
            libogg