Port from qAsConst() to std::as_const()

We've been requiring C++17 since Qt 6.0, and our qAsConst use finally
starts to bother us (QTBUG-99313), so time to port away from it
now.

Since qAsConst has exactly the same semantics as std::as_const (down
to rvalue treatment, constexpr'ness and noexcept'ness), there's really
nothing more to it than a global search-and-replace.

Task-number: QTBUG-99313
Change-Id: I88edd91395849574436299b8badda21bb93bea39
Reviewed-by: hjk <hjk@qt.io>

src/plugins/help/docsettingspage.cpp

@@ -327,7 +327,7 @@ void DocSettingsPageWidget::removeDocumentation(const QList<QModelIndex> &items)
     Utils::sort(itemsByDecreasingRow, [](const QModelIndex &i1, const QModelIndex &i2) {
         return i1.row() > i2.row();
     });
-    for (const QModelIndex &item : qAsConst(itemsByDecreasingRow)) {
+    for (const QModelIndex &item : std::as_const(itemsByDecreasingRow)) {
         const int row = item.row();
         const QString nameSpace = m_model.entryAt(row).nameSpace;