Use Jackson as the default JSON implementations, update libraries

[swallez]

Our docs illustrate/promote using Jackson as a JsonMapper implementation, and yet it was an optional dependency.

This PR makes Jackson the default implementation, and makes it a required dependency. It also updates all JSON-related libraries to their latest version.

[l-trotta]

we're about to remove the serverless docs, we can merge #973 before this one

[swallez]

Yes, let's merge #973 and I'll rebase this PR.

[wilkinsona]

This change means that elasticsearch-rest-client is no longer on the consumer's classpath by default. Was that intentional? It's a breaking change in some situations and seems unrelated to using Jackson as the default JSON implementation.

[l-trotta]

Hey @wilkinsona, the elasticsearch-rest-client dependency change is explained more in depth in the release notes: https://www.elastic.co/docs/release-notes/elasticsearch/clients/java/9-0-0.

If you need additional info let me know! We tried making the release highlights as detailed as possible this time around, but maybe there are use cases we didn't consider.

[l-trotta]

also beware of the known issues: https://www.elastic.co/docs/release-notes/elasticsearch/clients/java/known-issues#known-issues-9-0-0 and please use the latest patch 9.0.3 where said issue is fixed :)

[wilkinsona]

Thanks, @l-trotta. I only noticed the change upon a 9.0.2 to 9.0.3 upgrade. I guess that change must have shaken something else loose if the move to optional was in earlier 9.0.x releases. We've added a dependency on the old rest client for now and plan to switch to the new one.

[l-trotta]

That is great to hear! Feel free to ping me if any help or additional explanation is needed