Previously, unlike embedded Jetty, Netty, and Tomcat, Undertow would
not stop when one of its worker threads was in use. This meant that a
a long-running or stalled request could prevent the application from
shutting down in response to SIGTERM or SIGINT, and SIGTERM would be
required to get the process to exit.

This commit updates the factories for the reactive and servlet
Undertow web server factories to configure Undertow to use a 0ms
shutdown timeout. This aligns it with the behaviour of Jetty, Netty,
and Tomcat. Tests have been introduced to verify the behaviour across
the reactive and servlet variants of all four supported embedded web
servers.

Fixes gh-21319