Overview Alerts Histogram stacking defaults to signal.rule.name

Since this is now the default for all AlertsHistograms, I've moved this
default upstream into the histogram itself.

x-pack/plugins/security_solution/public/detections/components/alerts_histogram_panel/index.tsx

@@ -80,10 +80,14 @@ const getHistogramOption = (fieldName: string): MatrixHistogramOption => ({
 
 const NO_LEGEND_DATA: LegendItem[] = [];
 
+const DEFAULT_STACK_BY = 'signal.rule.name';
+const getDefaultStackByOption = (): AlertsHistogramOption =>
+  alertsHistogramOptions.find(({ text }) => text === DEFAULT_STACK_BY) ?? alertsHistogramOptions[0];
+
 export const AlertsHistogramPanel = memo<AlertsHistogramPanelProps>(
   ({
     chartHeight,
-    defaultStackByOption = alertsHistogramOptions[8], // signal.rule.name
+    defaultStackByOption = getDefaultStackByOption(),
     deleteQuery,
     filters,
     headerChildren,

x-pack/plugins/security_solution/public/overview/components/signals_by_category/index.tsx

@@ -17,7 +17,6 @@ import { UpdateDateRange } from '../../../common/components/charts/common';
 import { GlobalTimeArgs } from '../../../common/containers/use_global_time';
 
 const DEFAULT_QUERY: Query = { query: '', language: 'kuery' };
-const DEFAULT_STACK_BY = 'signal.rule.threat.tactic.name';
 const NO_FILTERS: Filter[] = [];
 
 interface Props extends Pick<GlobalTimeArgs, 'from' | 'to' | 'deleteQuery' | 'setQuery'> {
@@ -62,13 +61,9 @@ const SignalsByCategoryComponent: React.FC<Props> = ({
     [setAbsoluteRangeDatePicker]
   );
 
-  const defaultStackByOption =
-    alertsHistogramOptions.find((o) => o.text === DEFAULT_STACK_BY) ?? alertsHistogramOptions[0];
-
   return (
     <AlertsHistogramPanel
       deleteQuery={deleteQuery}
-      defaultStackByOption={defaultStackByOption}
       filters={filters}
       from={from}
       headerChildren={headerChildren}