Closes gh-13830

This commit adds a new `WebFluxRegistrations` interface that
allows developers to register custom instances of key WebFlux
infrastructure components, such as `RequestMappingHandlerMapping`
and `RequestMappingHandlerAdapter`.

Closes gh-13997

Because `HttpPutFormContentFilter` has been deprecated in Spring
Framework 5.1, this commit updates the auto-configuration to replace it
with the new `FormContentFilter`. This new filter is building on the
previous one and supports HTTP DELETE requests as well.

Both filters should not be used in addition, so the former
configuration has been removed. This commit also adds configuration
metadata to let developers know about the configuration key change.

Closes: gh-13363

Closes gh-14028

See gh-14028

See gh-14028

The Spring Boot integration is now handled by the Narayana project
itself at https://github.com/snowdrop/narayana-spring-boot

This commit removes our support.

Closes gh-12026

Closes gh-14120

Closes gh-14119

Closes gh-14118

Closes gh-14117

Closes gh-14116

Closes gh-14115

Closes gh-14114

Closes gh-14113

Closes gh-14112

Closes gh-14111

Closes gh-14110

Closes gh-14109

Closes gh-14108

Closes gh-14107

Closes gh-14106

Closes gh-14105

Closes gh-14104

Closes gh-14103

Closes gh-14102

Closes gh-14101

Closes gh-14100

Closes gh-14099

Closes gh-14008

The `shouldHandleFailure` configuration option has been removed from
`DispatcherServlet`.

See gh-13511

Closes gh-14038

Closes gh-14098

Closes gh-14097

Closes gh-14096

Closes gh-14095