Fix /admin/tags/:id crashing since Rails 6.1 update (#15953)

Raw SQL passed to `pluck` now has to be explicitly marked as SQL via
Arel.sql, see https://github.com/rails/rails/pull/27947

spec/controllers/admin/tags_controller_spec.rb

@@ -20,4 +20,16 @@ RSpec.describe Admin::TagsController, type: :controller do
       expect(response).to have_http_status(200)
     end
   end
+
+  describe 'GET #show' do
+    let!(:tag) { Fabricate(:tag) }
+
+    before do
+      get :show, params: { id: tag.id }
+    end
+
+    it 'returns status 200' do
+      expect(response).to have_http_status(200)
+    end
+  end
 end