V23: Cannot add a custom VaadinServlet in plain servlet starter

Description of the bug

Tried the V23 skeleton-starter-flow by adding a custom VaadinServlet class and it crashes the application in a way that requesting http://localhost:8080/ would result in listing root directory contents instead of loading the MainView. It seems that having "/VAADIN/*" is a must with the latest versions, but the other custom urlPattern such as "/vaadin/*" or "/asdf/*" is arbitrary. Basically, the presence of any custom VaadinServlet is enough to break the application’s normal behavior. Adding a HttpServlet is fine and works as expected.

Minimal reproducible example

1. Download https://github.com/vaadin/skeleton-starter-flow/archive/v23.zip and unzip
2. Add a custom VaadinServlet class:

import com.vaadin.flow.server.VaadinServlet;
import javax.servlet.annotation.WebServlet;

@WebServlet(asyncSupported = true, urlPatterns = { "/vaadin/*", "/VAADIN/*"})
public class WebComponentVaadinServlet extends VaadinServlet {
}

1. Run: mvn clean jetty:run
2. Hit the http://localhost:8080/

Expected behavior

To see the MainView loaded.

Actual behavior

It lists root directory contents:

Versions:

- Vaadin / Flow version: Platform v23.1.4 and v23.2.0.alpha3  
- Java version: 17
- OS version: MacOs
- Browser version (if applicable): Firefox 102.0.1 (64-bit), Chrome 103.0.5060.134 (Official Build) (x86_64)
- Application Server (if applicable): Jetty
- IDE (if applicable): N/A

[mcollovati]

My opinion is that if a user configures Vaadin to work on a mapping different from root, Vaadin should do nothing to prevent listing. With jetty (but this holds for other containers) it is the default servlet that handles listing So if the Vaadin servlet is not mapped to root, IMO it is up to the user to configure the servlet container to avoid listing.

[mcollovati]

If we define a custom servlet with a mapping different from root, I think it is expected not to see the main view at http://localhost:8080/. I expect to see it for example at http://localhost:8080/asdf/ if mapping is /asdf/*, /VAADIN/* And in this case it works for me