TimeSeriesPanel: Fixed default value for gradientMode (#30484) (#30492)

(cherry picked from commit 59ef36812e)

Co-authored-by: Torkel Ödegaard <torkel@grafana.org>

public/app/plugins/panel/timeseries/config.ts

@@ -89,7 +89,7 @@ export function getGraphFieldConfig(cfg: GraphFieldConfig): SetFieldConfigOption
         .addRadio({
           path: 'gradientMode',
           name: 'Gradient mode',
-          defaultValue: graphFieldOptions.fillGradient[0],
+          defaultValue: graphFieldOptions.fillGradient[0].value,
           settings: {
             options: graphFieldOptions.fillGradient,
           },