Observability onboarding: Show beta integrations (#230914)

Currently, the search on the "Add data" page is not showing beta
integrations. This is problematic because we want to show the
content-only "content pack" integrations there as well.

With this PR this is the case:
<img width="1168" height="287" alt="Screenshot 2025-08-07 at 09 43 16"
src="https://github.com/user-attachments/assets/721da7a1-1fda-4610-866b-3e9ce7a4f404"
/>

Author: flash1293

x-pack/solutions/observability/plugins/observability_onboarding/public/application/package_list_search_form/package_list_search_form.tsx

@@ -48,7 +48,7 @@ const PackageListGridWrapper = ({
   excludePackageIdList = [],
 }: WrapperProps) => {
   const { filteredCards: integrationCards, isLoading } = useAvailablePackages({
-    prereleaseIntegrationsEnabled: false,
+    prereleaseIntegrationsEnabled: true,
   });
   const rewriteUrl = useCardUrlRewrite({ category: flowCategory, search: searchQuery });