Darken the IQR region

When the interquartile range is narrow, it's hard to see against the
median line, so I darkened another shade in USWDS's color tokens to make
it stand out a little more.

src/unified_graphics/static/js/component/ChartTimeSeries.js

@@ -54,7 +54,7 @@ export default class ChartTimeSeries extends ChartElement {
   }
 
   #iqr {
-    fill: #dfe1e2;
+    fill: #a9aeb1;
   }
 
   #current,