Loki: Fix vertical alignment issue in label selector (#18943)

- upgraded rc-cascader (did not fix the alignment issue)
- Safari seems to default to middle alignment, added override to explore
styles
- added empty expand icon property to satisfy API after upgrade

public/app/plugins/datasource/loki/components/LokiQueryFieldForm.tsx

@@ -165,6 +165,7 @@ export class LokiQueryFieldForm extends React.PureComponent<LokiQueryFieldFormPr
               options={logLabelOptions}
               onChange={this.onChangeLogLabels}
               loadData={onLoadOptions}
+              expandIcon={null}
               onPopupVisibleChange={(isVisible: boolean) => {
                 if (isVisible && onLabelsRefresh) {
                   onLabelsRefresh();