Increase contrast of search icon

This is no longer being flagged by Wave, but there was still
a low contrast between the search icon and its background.

Using dark-blue gives us a 9.37:1 contrast ratio which is easier
to perceive.

scss/components/_search.scss

@@ -72,7 +72,7 @@
 
   button {
     background-color: #d2e2f1;
-    color: govuk-colour("blue");
+    color: govuk-colour("dark-blue");
 
     &:hover {
       background-color: lighten(#d2e2f1, 5%);