fix: subheader should show as subtitle (#33172)

superset-frontend/plugins/plugin-chart-echarts/src/BigNumber/sharedControls.ts

@@ -129,5 +129,8 @@ export const subtitleControl: CustomControlItem = {
     label: t('Subtitle'),
     renderTrigger: true,
     description: t('Description text that shows up below your Big Number'),
+    mapStateToProps: state => ({
+      value: state.form_data.subheader || state.form_data.subtitle,
+    }),
   },
 };