Update dot style for DORA lead time graphs

This commit updates the data point style on lead time graphs (both
project and group-level) to always render a blue dot. This makes it
easier to see isolated data points on the graphs.

Changelog: changed
EE: true

ee/app/assets/javascripts/dora/components/util.js

@@ -116,6 +116,9 @@ export const buildNullSeriesForLeadTimeChart = (seriesData) => {
   });
 
   merge(nonNullSeries, {
+    showSymbol: true,
+    showAllSymbol: true,
+    symbolSize: 8,
     lineStyle: {
       color: dataVizBlue500,
     },

ee/spec/frontend/dora/components/__snapshots__/util_spec.js.snap

@@ -86,6 +86,9 @@ Array [
       "color": "#5772ff",
     },
     "name": "Lead time",
+    "showAllSymbol": true,
+    "showSymbol": true,
+    "symbolSize": 8,
   },
 ]
 `;

ee/spec/frontend/dora/components/util_spec.js

@@ -66,6 +66,9 @@ describe('ee/dora/components/util.js', () => {
         },
         {
           name: 'Chart title',
+          showAllSymbol: true,
+          showSymbol: true,
+          symbolSize: 8,
           data: expect.any(Array),
           lineStyle: {
             color: expect.any(String),