Previously, the default servlet was registered automatically when using
embedded Jetty, Tomcat, or Undertow. However, it is not used by the
majority of applications where Spring MVC's DispatcherServlet will be
the only servlet that's needed. As such configuring the default servlet
was wasting CPU and memory.

This commit changes the default for registering the default servlet to
false. It can be re-enabled by setting
server.servlet.register-default-servlet=true.

Closes gh-22915