qt: Fix deprecated QCharRef usage

Port bitcoin/bitcoin#18101 to
our codebase. Without this commit every Alt-Tab or other
keypress on the SplashScreen will lead to the following
message in debug.log:

GUI: Using QCharRef with an index pointing outside the valid range of a QString. The corresponding behavior is deprecated, and will be changed in a future version of Qt.

src/qt/splashscreen.cpp

@@ -144,7 +144,7 @@ SplashScreen::~SplashScreen()
 bool SplashScreen::eventFilter(QObject * obj, QEvent * ev) {
     if (ev->type() == QEvent::KeyPress) {
         QKeyEvent *keyEvent = static_cast<QKeyEvent *>(ev);
-        if(keyEvent->text()[0] == 'q') {
+        if (keyEvent->key() == Qt::Key_Q) {
             StartShutdown();
         }
     }