Upgrade GDAL, PROJ and GEOTIFF in Superbuild
At some point we should upgrade GDAL version in the Superbuild to version 3.X.
Currently GDAL 3.X is supported (see !511 (merged)), but the Superbuild version is 2.4.1. Upgrading GDAL to version 3.X also means we have to upgrade PROJ to version 6.0 and GEOTIFF to a recent version e.g. 1.5.1 (because GEOTIFF depends on PROJ). Note that we don't have to upgrade OSSIM (fortunately because there is no new version) because it depends on GEOTIFF, but not directly on PROJ.