Search service incorrectly uses elasticsearch when indexing, but not search, is enabled

Originally spotted in https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/6587

::Project has a use_elasticsearch? method that is designed to tell us whether we should index the project or not. However, the project search service uses it unconditionally to decide whether to search using elasticsearch or not:

      # Project
      def use_elasticsearch?
        ::Gitlab::CurrentSettings.elasticsearch_indexes_project?(self)
      end

      # ApplicationSetting
      def elasticsearch_indexes_project?(project)
        return false unless elasticsearch_indexing?
        return true unless elasticsearch_limit_indexing?

        elasticsearch_limited_projects.exists?(project.id)
      end

      # Search::ProjectService
      def use_elasticsearch?
        project.use_elasticsearch?
      end

The same problem exists in group, but not global, search.

Since we need this to work properly before we can use elasticsearch on GitLab.com, I'll assign this to myself and %12.0 . cc @DouweM @phikai @mdelaossa