Purge Qt Declarative's use of QDateTime's locale-dependent APIs

QDateTime shall drop its support for locale-dependence and produce pure C-locale results at Qt 6 (see QTBUG-80441). All locale-dependence in date-time APIs should, thereafter, be mediated by use of QLocale, to which QDateTime has long delegated these conversions.
As part of this I plan to deprecate the locale-specific enum members in Qt::DateFormat; at the very least, the QDateTime APIs shall stop supporting them.

Unfortunately, Qt Declarative has its own uses of Qt::DateFormat, e.g. qtdeclarative/src/qml/qml/v8/qqmlbuiltinfunctions.cpp defines QtObject::method_formatDateTime() and related formatDate(), formatTime() methods; these interpret a format argument as Qt::DateFormat, which they then forward to QDateTime::toString().
They shall, at the very least, need to replace this last call with a switch on Qt::DateFormat, that calls QLocale::toString() for the locale-dependent formats, or QDateTime::toString() for the others.