Issue: SPR-14906

Issue: SPR-14998

Issue: SPR-14940

Due to a few changes in `WebSocketServerFactory` and `Session` API, our
`JettyRequestUpgradeStrategy` and `JettyWebSocketSession` needed to
adapt. As of 9.3.15+ and 9.4.0+, some reflection is required to support
previous versions.

Spring 5.0 supports Jetty 9.3 and 9.4.

Issue: SPR-14940

This reverts commit 472126c0.

Issue: SPR-14940

Issue: SPR-14527

Issue: SPR-14924

This commit polishes Kotlin nullable support by reusing
MethodParameter#isOptional() instead of adding a new
MethodParameter#isNullable() method, adds
Kotlin tests and introduces Spring Web Reactive
support.

Issue: SPR-14165

Where `isOptional` is used, also check for `isNullable` i.e.
values are not considered required if they are Kotlin nullables:
- spring-messaging: named value method arguments
- spring-web: named value method arguments
- spring-webmvc: request parts

This means that Kotlin client code no longer has to explicity specify
"required=false" for Kotlin nullables -- this information is inferred
automatically by the framework.

Issue: SPR-14165

Includes basic abstractions and an RxNetty support to start.

Issue: SPR-14527

Since https://jira.sonarsource.com/browse/SONARGRADL-30, we can now
safely update to sonarqube plugin v2.2.1.

The protobuf gradle plugin is now deprecated - instead of configuring
the new plugin supported by Google, this commits drops that part of the
gradle build since that proto->java conversion is rarely needed and can
be manually run by any member of the team.

Issue: SPR-14880

 - ScriptedSubscriber has been renamed to Verifier
 - The Publisher is passed to create() instead of verify()
 - No more need to specify the generic type explicitly
 - Version is now sync with reactor-core

Issue: SPR-14800

Reactor recently added the `ScriptedSubscriber` in its new
`reactor-addons` module. This `Subscriber` revissits the previous
`TestSubscriber` with many improvements, including:

* scripting each expectation
* builder API that guides you until the final verification step
* virtual time support

This commit refactor all existing tests to use this new
infrastructure and removed the `TestSubscriber` implementation.

Issue: SPR-14800