Bump minimum Qt version (to build Qt Creator) to 5.11

Recent adaptations to Qt 5.13's API deprecations require using APIs
which were introduces in Qt 5.11.

Change-Id: I6c077d824c9ce716e019543b290c355a5d512fad
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/clangcodemodel/clangdiagnostictooltipwidget.cpp

@@ -386,14 +386,10 @@ private:
 
     static int widthLimit()
     {
-#if QT_VERSION >= QT_VERSION_CHECK(5, 10, 0)
         auto screen = QGuiApplication::screenAt(QCursor::pos());
         if (!screen)
             screen = QGuiApplication::primaryScreen();
         return screen->availableGeometry().width() / 2;
-#else
-        return QApplication::desktop()->availableGeometry(QCursor::pos()).width() / 2;
-#endif
     }
 
 private: