Updated the list of supported platforms in Doxygen

GaudiRelease/doc/doxygen/mainpage.md.in

@@ -16,8 +16,9 @@ the [list of classes](annotated.html) or via the [class hierarchy](hierarchy.htm
     available in the [detailed release notes](@ref release_notes_main)<br/>
     The [project release notes](release.notes.html) contain a summary of changes
     since the previous release.</td></tr>
-<tr><th>Supported Platforms</th><td>Scientific Linux CERN 6 with gcc-4.8 (released with debug and non debug
-    versions, 64 bits only, with C++11 enabled)</td></tr>
+<tr><th>Supported Platforms</th><td>Scientific Linux CERN 6 and CentOS 7 with
+    gcc 4.9, 6 and 7 (released with debug and non debug
+    versions, 64 bits only, with C++14 enabled)</td></tr>
 <tr><th>Download</th><td>[Sources (on gitlab.cern.ch)][sources]</td></tr>
 <tr><th>Documentation</th><td>%Gaudi Developers Guide [v9][gug-html]
     ([.pdf][gug-pdf], [.pdf 2up (for printing)][gug-2up-pdf])</td></tr>