[applets/digital-clock] Set a maximum height for the timezone scrollview

Otherwise when there are tons and tons of timezones, the page becomes
scrollable rather than the the Scrollview becoming scrollable.

Fixes a recent regression introduced by
https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/821

CCMAIL: isma.af@gmail.com

applets/digital-clock/package/contents/ui/configTimeZones.qml

@@ -50,6 +50,10 @@ ColumnLayout {
     QQC2.ScrollView {
         Layout.fillWidth: true
         Layout.fillHeight: true
+        // Or else the page becomes scrollable when the list has a lot of items,
+        // rather than the list becoming scrollable, which is what we want
+        Layout.maximumHeight: timeZonesPage.parent.height - Kirigami.Units.gridUnit * 7
+
         Component.onCompleted: background.visible = true // enable border
 
         // HACK: Hide unnecesary horizontal scrollbar (https://bugreports.qt.io/browse/QTBUG-83890)