Upgrade CDN version of jQuery

https://github.com/django-oscar/django-oscar/pull/3018 updated the in-tree versions

src/oscar/templates/oscar/base.html

@@ -35,7 +35,7 @@
         {# Scripts loaded from a CDN. #}
         {% block cdn_scripts %}
             <!-- jQuery -->
-            <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
+            <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>
             <script>window.jQuery || document.write('<script src="{% static "oscar/js/jquery/jquery.min.js" %}"><\/script>')</script>
         {% endblock %}