Servlet filter mappings not working with quickstart

**Jetty version(s)**
12.0.x+

**Jetty Environment**
ee8, ee9, ee10, ee11

**Description**
When using quickstart on a web application using Servlet filters, all the `<dispatcher>` tags are lost when the quickstart-web.xml is generated.

see https://github.com/GoogleCloudPlatform/appengine-java-standard/issues/423