Refactor `ReactiveWebApplicationContext` implementations to align closer
with the `WebApplicationContext` implementations defined in
Spring Framework.

The following classes are now provided:

  - `AnnotationConfigReactiveWebApplicationContext` -- A refreshable
    reactive web context with support for `@Configuration` classes.

  - `GenericReactiveWebApplicationContext` -- A non-refreshable reactive
    GenericApplicationContext.

  - `ReactiveWebServerApplicationContext` --  A non-refreshable reactive
    GenericApplicationContext with support for server discovery.

  - `AnnotationConfigReactiveWebServerApplicationContext` -- A
    non-refreshable reactive `GenericApplicationContext` with support
    for `@Configuration` classes and server discovery.

These classes roughly align to the following Servlet equivalents:

  - `AnnotationConfigWebApplicationContext` (Spring Framework)
  - `GenericWebApplicationContext` (Spring Framework)
  - `ServletWebServerApplicationContext` (Spring Boot)
  - `AnnotationConfigServletWebServerApplicationContext` (Spring Boot)

An additional `ConfigurableReactiveWebEnvironment` interface as also
been introduced, primarily for `@ConditionalOnWebApplication` to use.

Fixes gh-10852

Fixes gh-10866

Closes gh-10920

Closes gh-10919

Closes gh-10918

Closes gh-10917

Closes gh-10916

Closes gh-10915

Closes gh-10914

Closes gh-10913

Closes gh-10912

Closes gh-10911

Closes gh-10910

Closes gh-10909

Closes gh-10908

Closes gh-10905

This commit ensures that all errors handled by the
`DefaultErrorWebExceptionHandler` (Spring WebFlux error convetion
support) logs an error with request information and exception
stacktrace.

This is limited to errors that result in an HTTP 5xx error.
Exceptions that extend `ResponseStatusException` and set a non-5xx
status will not be logged.

Closes gh-10904

On WebSphere Liberty, the javax.validation package may be visible but
other javax.validation packages upon which it depends may not be
visible. This can lead to an incorrect assumption that the Bean
Validation API is available when, in fact, it is only partially
available.

Add an additional check for the availability of a class in the
javax.validation.bootstrap package to ensure that it's not just the
javax.validation package that's available.

Closes gh-10877

This commit enables HTTP/2 support for Tomcat and Undertow, for both
Servlet-based and Reactive applications.

Enabling the `server.http2.enabled` configuration flag is enough with
Undertow.

Tomcat has a few prerequisites:

* Tomcat 8.5 requires JDK8+ and the proper libtcnative version installed
on the host
* Tomcat 9.0.x requires JDK9+

Closes gh-10043

This commit adds a new configuration properties class for configuring
HTTP/2 protocol support.
By default, this protocol is disabled as enabling it requires several
manual changes:

* configuring a web server for proper TLS and ALPN support
* configuring a proper SSL certificate

See gh-10043

This commit applies the SSL configuration to the following reactive web
servers: Jetty, Tomcat, Undertow, Reactor Netty.

Closes gh-9431

This commits adds the AprLifecycleListener as a default
LifecycleListener in order to detect and use automatically the
tomcatnative library if it is available, for SSL support.

This feature can be useful for both performance reasons or for
supporting ALPN when using JDK8.

See gh-10043
Closes gh-9964

See gh-10884

* pr/10892:
  Format production-ready-features.adoc to 90 chars
  Format spring-boot-features.adoc to 90 chars
  Format using-spring-boot.adoc to 90 chars
  Format getting-started.adoc to 90 chars

Remove extraneous white space and adjusted the length of non-code lines
to be as close to 90 characters (but not over 90 characters) as
possible.

Update a couple things I missed in the editing pass, too.

Closes gh-10897

Remove extraneous white space and adjusted the length of non-code lines
to be as close to 90 characters (but not over 90 characters) as
possible.

Update a couple things I missed in the editing pass, too.

Closes gh-10896

Remove extraneous white space and adjusted the length of non-code lines
to be as close to 90 characters (but not over 90 characters) as
possible.

Update a couple things I missed in the editing pass, too.

Closes gh-10893

Remove extraneous white space and adjusted the length of non-code lines
to be as close to 90 characters (but not over 90 characters) as
possible.

Update a couple things I missed in the editing pass, too.

Closes gh-10892

* pr/10801:
  Restore execution permission on launch.script

Restore the execute permission that was accidentally removed in
commit 5ee28a08.

Closes gh-10801

* pr/10740:
  Support custom SpringBootContextLoader environment

* pr/8516:
  Introduce configurable logging.pattern.dateformat

Provide a `getEnvironment` method in `SpringBootContextLoader` to allow
specialized `ConfigurableEnvironment` implementations to be used.

Closes gh-10740

Add configurable logger dateformat support for Logback and Log4j2.

Closes gh-8516