Add missing since to QEvent::copy docs

Change-Id: Idfadc1801b0e97a5030d2784a65ae658900efd12
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit 9b2df27bb3a5e8fb788f7274013aa66d32d1250d)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/kernel/qcoreevent.cpp

@@ -355,6 +355,7 @@ QEvent::~QEvent()
 
 /*!
     Creates and returns an identical copy of this event.
+    \since 6.0
 */
 QEvent *QEvent::clone() const
 { return new QEvent(*this); }