Update required FontConfig version to 2.6

The information in this list is severely outdated. Qt depends on
FC_LCD_FILTER, which was introduced in FC 2.6, which was released
in 2008.

Change-Id: I5a74a3d60106f613033361814c4f183fb56484be
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

doc/src/platforms/linux.qdoc

@@ -211,7 +211,7 @@
     \li libfontconfig
     \li Font customization and configuration
     \li \tt{-fontconfig} or auto-detected
-    \li 2.1
+    \li 2.6
     \row {id="OptionalColor"}
     \li FreeType
     \li libfreetype