applets/digital-clock: use ColumnLayout to fix crash in Qt6

When parent Item uses childRect and the child uses fillWidth, it crashes.

Backtrace

#0  0x0000000000000000 in  ()
#1  0x00007fc2e1159e69 in QMetaObject::cast(QObject const*) const () at /lib64/libQt6Core.so.6
#2  0x00007fc2a81e6cf3 in QQuickLayout::effectiveSizePolicy_helper(QQuickItem*, Qt::Orientation, QQuickLayoutAttached*) () at /lib64/libQt6QuickLayouts.so.6
#3  0x00007fc2e1a7e6e3 in QGridLayoutItem::effectiveMaxSize(QSizeF const&) const () at /lib64/libQt6Gui.so.6
#4  0x00007fc2e1a7e831 in QGridLayoutItem::geometryWithin(double, double, double, double, double, QFlags<Qt::AlignmentFlag>, bool) const () at /lib64/libQt6Gui.so.6
#5  0x00007fc2e1a856fc in QGridLayoutEngine::setGeometries(QRectF const&, QAbstractLayoutStyleInfo const*) () at /lib64/libQt6Gui.so.6
#6  0x00007fc2a81e8740 in QQuickGridLayoutBase::rearrange(QSizeF const&) () at /lib64/libQt6QuickLayouts.so.6
#7  0x00007fc2a81e3822 in QQuickLayout::geometryChange(QRectF const&, QRectF const&) () at /lib64/libQt6QuickLayouts.so.6
#8  0x00007fc2e38118e6 in QQuickItem::setX(double) (this=0x78b4fb0, v=351) at /usr/src/debug/qtdeclarative-everywhere-src-6.4.2/src/quick/items/qquickitem.cpp:6978
#9  0x00007fc2e38234cf in QQuickItem::qt_metacall(QMetaObject::Call, int, void**) (this=0x78b4fb0, _c=QMetaObject::WriteProperty, _id=4, _a=0x7fffec8192b0) at /usr/src/debug/qtdeclarative-everywhere-src-6.4.2/build/src/quick/Quick_autogen/include/moc_qquickitem.cpp:1461
#10 0x00007fc2a81e8285 in QQuickLayout::qt_metacall(QMetaObject::Call, int, void**) () at /lib64/libQt6QuickLayouts.so.6
#11 0x00007fc2a81e8595 in QQuickGridLayoutBase::qt_metacall(QMetaObject::Call, int, void**) () at /lib64/libQt6QuickLayouts.so.6
#12 0x00007fc2a81e9ca5 in QQuickLinearLayout::qt_metacall(QMetaObject::Call, int, void**) () at /lib64/libQt6QuickLayouts.so.6
#13 0x00007fc2e21dc2f7 in QV4::QObjectWrapper::setProperty(QV4::ExecutionEngine*, QObject*, QQmlPropertyData const*, QV4::Value const&) () at /lib64/libQt6Qml.so.6
#14 0x00007fc2e21dd63e in QV4::QObjectWrapper::setQmlProperty(QV4::ExecutionEngine*, QQmlRefPointer<QQmlContextData> const&, QObject*, QV4::String*, QV4::QObjectWrapper::RevisionMode, QV4::Value const&) () at /lib64/libQt6Qml.so.6
#15 0x00007fc2e21e0462 in QV4::QObjectWrapper::virtualPut(QV4::Managed*, QV4::PropertyKey, QV4::Value const&, QV4::Value*) () at /lib64/libQt6Qml.so.6
#16 0x00007fc2e21bd711 in QV4::Lookup::setterFallback(QV4::Lookup*, QV4::ExecutionEngine*, QV4::Value&, QV4::Value const&) () at /lib64/libQt6Qml.so.6
#17 0x00007fc2a8a4cbc4 in  ()
#18 0x00007fffec819700 in  ()
#19 0x0000000000000000 in  ()