Closes gh-22612

Closes gh-22486

Closes gh-22588

This commit incoporates the remarks made during the Servlet.fn review.

See gh-21490

This commit adds support for HandlerFunctions and RouterFunctions to
DispatcherServlet, in the form of a HandlerAdapter and HandlerMapping.

See gh-21490

This commit adds tests for Servlet.fn, the servlet-based version of
HandlerFunctions and RouterFunctions.

See gh-21490

This commit introduces Servlet.fn, a servlet-based version of
WebFlux.fn, i.e. HandlerFunctions and RouterFunctions.

This commit contains the web framework only, further commits provide
tests and DispatcherServlet integration.

See gh-21490

Extend the list of disconnected client errors in HttpWebHandlerAdapter
to include the Reactor Netty AbortedException as well exceptions with
the message "connection reset by peer".

Closes gh-21790

Closes gh-22267

Closes gh-21874

See gh-22367

Prior to this commit, documentation for WebJar support referenced the
webjars-locator artifact; however, Spring now uses the
webjars-locator-core artifact.

This commit updates the documentation to reflect this.

Closes gh-22613

This commit changes the UndertowServerHttpResponse to use a
listener-based approach instead of a blocking approach.

Closes gh-22413

Closes gh-22308

Use the checkpoint operator at various places in WebFlux to insert
information that Reactor then uses to enrich exceptions, via suppressed
exceptions, when error signals flow through the operator.

Closes gh-22105

See gh-19975

See gh-19975

See gh-19975

This commit turns Coroutines suspending methods to `Mono` which can be
handled natively by WebFlux.

See gh-19975

See https://github.com/checkstyle/checkstyle/blob/master/config/suppressions.xml

See http://checkstyle.sourceforge.net/config.html#Configuration_XML_Structure

* pr/22597:
  Polish "Remove outdated link in documentation"
  Remove outdated link in documentation

Closes gh-22597

See gh-22597

Closes gh-22591