Fix Jetty as webserver documentation

[mhalbritter]

In https://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#howto.webserver.use-another we document how to use Jetty instead of Tomcat.

This contains

<servlet-api.version>3.1.0</servlet-api.version>

which is out of date. It also talks about

NOTE: The version of the servlet API has been overridden as, unlike Tomcat 9 and Undertow 2, Jetty 9.4 does not support servlet 4.0.
If you wish to use Jetty 10, which does support servlet 4.0, you can do so as shown in the following example:

which is out of date. We should update or remove this section.

[wilkinsona]

Duplicate of #33044?

[mhalbritter]

Definitely related. There's more in this section, for example the Tomcat versions, and outdated servlet versions, which should be fixed, too.

[mhalbritter]

Duplicate of #37238, fixed in b4721c9.