PAM .aux.xml and VRT: serialize Z component of a GCP as 'Z' attribute, for consistency, instead of GCPZ that could not be read back previously. In reading code, try reading 'Z' and if not found try 'GCPZ' (#5326)

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@26727 f0d54148-0727-0410-94bb-9a71ac55c965