Issue: SPR-16123

Issue: SPR-16117

This commit decouples ExchangeResult from knowledge about
WiretapClientHttpRequest/Response both of which are now private to
WiretapConnector.

ExchangeResult takes ClientHttpRequest/Response instead along with
promises for the serialized request and response content in the form
of MonoProcessor<byte[]>.

This sets up the possibility for an ExchangeResult to be created
outside and independent of WebTestClient, should we decide to make its
constructor public.

Issue: SPR-16101

Issue: SPR-16111

Includes upgrade to Hibernate Validator 6.0.4 (where applicable)

Issue: SPR-16115

The main way CONTRIBUTING will be viewed is on Github and the output
is just not good enough to justify using asciidoctor vs md/txt as
the recommended on the Github help page for contributing guidelines.

For example it was attractive to use a TOC and that works on Github
but doesn't look good. In the end it's a very short TOC and
maintaining manually is easy enough and it's easier to control the
look.

Essentially optimize for viewing on Github since the Asciidoctor
TOC works but doesn't look good with no styling.

The guidelines were originally created several years ago. This commit
does not fundamentally change anything but rather make the guidance
more compact.

The commit consolidates into into CONTRIBUTING.adoc information from
CONTRIBUTING-DOCUMENTATION.adoc and also the wiki page "The Lifecycle
of an Issue".

Issue: SPR-16108

Directly inlined hasLength implementations for proper nullability detection in IntelliJ, assuming a hasText checked value is never null afterwards. Since the JVM is going to do this at runtime anyway, this is effectively equivalent but more indicative for source code introspection algorithms.

Issue: SPR-15540

Issue: SPR-16107

Issue: SPR-16102

A new HTTP/2 support section has been added in the Spring MVC
documentation. Also, a dedicated wiki page is created for specific
container support information.

Issue: SPR-15821

Issue: SPR-16077

Issue: SPR-16103

Issue: SPR-16091

Issue: SPR-16044

This commit exposes the ClientRequest's URL and HttpMethod fields via a
setter, so that they can be changed more easily in a request that was
created via ClientRequest.from(ClientRequest).

Issue: SPR-16093