Rename site-search to site_search

To ensure that the event name for when a user searches is in keeping
the GA4 Event Naming Rules which state that underscores should be used
in event names.

src/javascripts/components/search.tracking.mjs

@@ -24,7 +24,7 @@ export function trackConfirm(searchQuery, searchResults, result) {
     .filter((product) => product.name === result.title)
 
   addToDataLayer({
-    event: 'site-search',
+    event: 'site_search',
     eventDetails: {
       category: 'site search',
       action: 'click',