Fix use of deprecated API in wayland/custom-extension example

 3982: /home/qt/work/qt/qtwayland/examples/wayland/custom-extension/client-common/customextension.cpp: In member function ‘virtual bool TestNamespace::CustomExtension::eventFilter(TestNamespace::QObject*, TestNamespace::QEvent*)’:
 /home/qt/work/qt/qtwayland/examples/wayland/custom-extension/client-common/customextension.cpp:86:33: warning: ‘const TestNamespace::QRegion& TestNamespace::QExposeEvent::region() const’ is deprecated: Handle QPaintEvent instead [-Wdeprecated-declarations]
    86 |         if (exposeEvent->region().isNull())
       |                                 ^
 In file included from /home/qt/work/install/include/QtGui/QPlatformSurfaceEvent:1,
                  from /home/qt/work/qt/qtwayland/examples/wayland/custom-extension/client-common/customextension.cpp:54:
 /home/qt/work/install/include/QtGui/qevent.h:567:27: note: declared here
   567 |     inline const QRegion &region() const { return m_region; }
       |                           ^~~~~~
 make[3]: Leaving directory '/home/qt/work/qt/qtwayland_qmake/examples/wayland/custom-extension/qml-client'