Fix PDF export size of elevation plots

QPdfWriter doesn't return the output resolution as the device
physicalDpiX(?!), so let's hack around this by just setting
the output resolution to match the value reported by
physicalDpiX...

Fixes qgis#57880

src/app/elevation/qgselevationprofilewidget.cpp

@@ -768,7 +768,7 @@ void QgsElevationProfileWidget::exportAsPdf()
   pageLayout.setMode( QPageLayout::FullPageMode );
   pdfWriter.setPageLayout( pageLayout );
   pdfWriter.setPageMargins( QMarginsF( 0, 0, 0, 0 ) );
-  pdfWriter.setResolution( 300 );
+  pdfWriter.setResolution( 1200 );
 
   QPainter p;
   if ( !p.begin( &pdfWriter ) )