pkg/ui: properly derive XScale in statement details page

Fixes: #121362

#118680 introduced code to
align the timeseries charts on the statement details page to the time
range set by the time picker, instead of the time range that we had
available data for.

The code contained a bug where it would set the start & end of the
XScale to `Moment.prototype.valueOf` instead of the result of *calling*
`Moment.prototype.valueOf()`. This causes the start & end timestamps to
be set to functions, instead of unix timestamps, which broke the charts.

This PR simply invokes the function.

Release note (bug fix): The timeseries graphs shown on the SQL Activity
statement details page in DB Console will now render properly, after
fixing a bug related to setting the time range of the charts.

pkg/ui/workspaces/cluster-ui/src/statementDetails/statementDetails.tsx

@@ -697,9 +697,9 @@ export class StatementDetails extends React.Component<
 
     const [chartsStart, chartsEnd] = toRoundedDateRange(this.props.timeScale);
     const xScale = {
-      graphTsStartMillis: chartsStart.valueOf,
-      graphTsEndMillis: chartsEnd.valueOf,
-    } as unknown as XScale;
+      graphTsStartMillis: chartsStart.valueOf(),
+      graphTsEndMillis: chartsEnd.valueOf(),
+    } as XScale;
 
     return (
       <>