Remove the need to run rake tasks to create index when enabling Elasticsearch integration

As you can see from !40062 (diffs) we need to first create an index before we can enable "Elasticsearch indexing". This check is done at https://gitlab.com/gitlab-org/gitlab/-/blob/44479af19b4f6bd3bf7cb40a14075b36642d4b6b/ee/app/services/ee/application_settings/update_service.rb#L49 .

We should simplify the user experience by simply doing a find or create index when they first enable "Elasticsearch indexing". This could save a couple of steps in the setup workflow.