Mark ES experimental indexer as beta

ee/app/views/admin/application_settings/_elasticsearch_form.html.haml

@@ -14,7 +14,7 @@
         .form-check
           = f.check_box :elasticsearch_experimental_indexer, disabled: missing, class: 'form-check-input'
           = f.label :elasticsearch_experimental_indexer, class: 'form-check-label' do
-            Use <a href="https://gitlab.com/gitlab-org/gitlab-elasticsearch-indexer">experimental repository indexer</a>
+            Use <a href="https://gitlab.com/gitlab-org/gitlab-elasticsearch-indexer">beta repository indexer</a>
             - if missing
               (not installed)