refactor: rename __bootstrap__ telemetry event to __start__

Aligns with the InstantSearch.start() method name.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: tkrugg

packages/instantsearch.js/src/middlewares/createInsightsMiddleware.ts

@@ -455,7 +455,7 @@ See documentation: https://www.algolia.com/doc/guides/building-search-ui/going-f
         instantSearchInstance.on('render', telemetryOnRender);
 
         sendTelemetryEvent({
-          eventName: '__bootstrap__',
+          eventName: '__start__',
           performance: {
             timeSincePageLoad:
               typeof performance !== 'undefined'