Re enable full build with jdk17

[pderop]

Since the migration to Antora, it's not possible anymore to do a full build using jdk17:

sdk use java 17.0.10-zulu
./gradlew publishToMavenLocal

> Configure project :
1.2.0-SNAPSHOT is a snapshot, will use 1.2.0.BUILD-202404040843 for bnd
[Spotless] Local run detected, ratchet from origin/main

> Task :docs:generatePomFileForMavenJavaPublication FAILED

FAILURE: Build failed with an exception.

* Where:
Script '/Users/***/Documents/reactor-netty/gradle/setup.gradle' line: 153

* What went wrong:
Execution failed for task ':docs:generatePomFileForMavenJavaPublication'.
> Could not apply withXml() to generated POM
   > Cannot invoke method children() on null object

* Try:
> Run with --stacktrace option to get the stack trace.
> Run with --info or --debug option to get more log output.
> Run with --scan to get full insights.

* Get more help at https://help.gradle.org/

This PR re enable full build using jdk17:

• in build.gradle: toolchain sets JDK17 for docs sub project
• in gradle/settings.gradle, the docs project must be excluded, it does not contain dependencies, that was causing the Cannot invoke method children() on null object error
• in reactor-netty/build.gradle: if JDK17 is used, then a dependency to `:docs:antora' is dynamically added for the docsZip task.

[pderop]

Rebased on top of latest main branch in order to manually merge #3140 which was conflicting with this PR.

[pderop]

@violetagg , thanks for your review!