Port to non-deprecated QLabel::pixmap()

Quoting from:
714409b23c

```
The QLabel changes to the pixmap/picture getters provide the following
migration path:

QPixmap *ppix = l->pixmap(); // up to 5.15, warns in 5.15
QPixmap pval = l->pixmap(Qt::ReturnByValue); // new in 5.15, works in 6
QPixmap pixmap = l->pixmap(); // from Qt 6 on
```

Since we can't require 5.15 yet in dolphin, the port is done only when
building with Qt >= 5.15

src/views/tooltips/dolphinfilemetadatawidget.cpp

@@ -81,9 +81,16 @@ void DolphinFileMetaDataWidget::setPreview(const QPixmap& pixmap)
 
 QPixmap DolphinFileMetaDataWidget::preview() const
 {
+#if QT_VERSION < QT_VERSION_CHECK(5, 15, 0)
     if (m_preview->pixmap()) {
         return *m_preview->pixmap();
     }
+#else
+    if (!m_preview->pixmap(Qt::ReturnByValue).isNull()) {
+        return m_preview->pixmap(Qt::ReturnByValue);
+    }
+#endif
+
     return QPixmap();
 }