Update minimum freetype version

We need at least 2.3.0 now (released 2007).

Task: QTBUG-70601
Change-Id: I62acb85ff17b821ed4f4fbd6d1436df24a76cb98
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

doc/src/platforms/linux.qdoc

@@ -217,7 +217,7 @@
     \li libfreetype
     \li Font engine
     \li
-    \li 2.2.0
+    \li 2.3.0
 
     \row {id="DefaultColor"}
     \li Xi