DEV: Make the Glimmer Search Menu the new default (#25092)

- Convert group based `experimental_search_menu_groups_enabled` site setting to be a _hidden_ boolean `experimental_search_menu` setting.
- Make default `true`
- Remove widget search menu tests

Discourse Encrypt Test Failure Fix - https://github.com/discourse/discourse-encrypt/pull/301

app/serializers/current_user_serializer.rb

@@ -66,8 +66,7 @@ class CurrentUserSerializer < BasicUserSerializer
              :sidebar_tags,
              :sidebar_category_ids,
              :sidebar_sections,
-             :new_new_view_enabled?,
-             :experimental_search_menu_groups_enabled?
+             :new_new_view_enabled?
 
   delegate :user_stat, to: :object, private: true
   delegate :any_posts, :draft_count, :pending_posts_count, :read_faq?, to: :user_stat