Scheduled List: Fix has-due? and has-start?

As [EDS added support for `has-end?`, `has-due?` and `has-duration?` in `3.39.1+`](https://gitlab.gnome.org/GNOME/evolution-data-server/-/issues/254), we should also fix our s-expression for the Scheduled List in [`MainWindow.vala`](https://github.com/elementary/tasks/blob/89f48c92a28af71d8b6c460483fc2daec9271a51/src/MainWindow.vala#L167):

```
var query = "AND (NOT is-completed?) (OR (has-start?) (has-alarms?))";
```

According to [RFC554, either dtstart or due](https://tools.ietf.org/html/rfc5545#section-3.6.2) can be set - so our s-expression should be changed to:

```
var query = "AND (NOT is-completed?) (OR (has-start?) (has-due?))";
```

@danrabbit will Evolution Data Server `3.39.1+` eventually land in Odin?