Drop Qt 5 support

Qt 6 has been available for almost 4 years. Qt 5 is no longer officially supported by Qt for opensource, it's time to drop Qt 5.

src/dialogs/snapdialog.cpp

@@ -30,11 +30,7 @@ SnapDialog::SnapDialog(QWidget *parent) : MessageDialog(parent) {
   setWindowTitle(tr("Strawberry is running as a Snap"));
 
   const QIcon icon = IconLoader::Load(QStringLiteral("dialog-warning"));
-#if (QT_VERSION >= QT_VERSION_CHECK(6, 0, 0))
   const QPixmap pixmap = icon.pixmap(QSize(64, 64), devicePixelRatioF());
-#else
-  const QPixmap pixmap = icon.pixmap(QSize(64, 64));
-#endif
   ui_->label_logo->setPixmap(pixmap);
 
   QString text;