__onChildValidatePerformed(ev) {
      if (ev && this.isRegisteredFormElement(/** @type {FormControl} */ (ev.target))) {
        this.validate();
      }
    }
Gets triggered by event 'validate-performed' which enabled us to handle 2 different situations
 - react on modelValue change, which says something about the validity as a whole
 (at least two checkboxes for instance) and nothing about the children's values
 - children validity states have changed, so fieldset needs to update itself based on that
@param {Event} ev