The `change_date` slot should be date-typed

The `change` node has a `change_date` slot, supposedly intended to represent the date when a change was created.

That slot has no explicit range:

```
slots:
[…]
change_date:
slot_uri: dcterms:date
[…]
```

so it defaults to be string-typed, which doesn’t seem to make much sense. It should have a range of `xsd:date` (or possibly `xsd:dateTime` to allow finer granularity than a day) instead.

Related: I’m unsure what to make of the following comment in the `slot_usage` section:

```
change_date:
comments:
- This should be the composition of 'was generated by' and 'ended at time'
```

What does that mean? Is that why the slot is not date-typed? Is the slot supposed to contain a string representation of a more complex structure?

Start by locating the schema definition for the change node and its slot_usage entry for change_date. Review how ranges and the existing comment are represented, then resolve whether the intended value is xsd:date or xsd:dateTime and update the schema and any affected validation tests.

Written by the indexing model from the issue text.