Update examples/clipboard_.py to fix FutureWarning (#6365)

Closes #6364

I've updated the example script to no longer access the clipboard via
`viewer.window.qt_viewer.clipboard` (which we plan to deprecate).
Instead we now use `viewer.window.clipboard` directly (which is part of
the external API).

Related:
* #3765
* #6283

examples/clipboard_.py

@@ -34,8 +34,8 @@ def create_grabber_widget():
     widget = Grabber()
 
     # connect buttons
-    widget.copy_canvas_btn.clicked.connect(lambda: viewer.window.qt_viewer.clipboard())
-    widget.copy_viewer_btn.clicked.connect(lambda: viewer.window.clipboard())
+    widget.copy_canvas_btn.clicked.connect(lambda: viewer.window.clipboard(canvas_only=True))
+    widget.copy_viewer_btn.clicked.connect(lambda: viewer.window.clipboard(canvas_only=False))
     return widget