Update docs to remove requirement for installing the integration

[endorama]

Problem

With #11529 we are moving away from using the integration as our main contributor to setup apm-server ingestion. Once that is completed and released our installation documentation will not be up to date.

Goal

Provide up to date documentation for ingesting data with apm-server using the apm-data plugin or the apm integration package.

Context

Currently the data ingestion setup with apm-server always relies on installing the apm integration package, responsible for all required setup steps.

Once #11529 is released we will be in a different scenario than today:

• all required Elasticsearch assets to enable ingestion will be managed by and through the apm-data Elasticsearch plugin
• running apm-server data ingestion component with Fleet server will be managed by the apm integration package (referred to as an "input-only" package because does not handle data streams and mappings)

We need to identify where in the documentation changes are required and apply them to ensure we provide up to date information.

e.g. at https://www.elastic.co/guide/en/observability/current/apm-getting-started-apm-server.html##

[axw]

As part of this we should also verify that the ILM docs are accurate. We now use DSLM by default, but I think the docs around customising ILM should still be accurate. We should update the instructions to guide users towards DSLM, but it's not urgent as long as the ILM method still works.

[simitt]

@endorama @axw IMO we updated the docs in a few iterations - do you still see docs updates missing?

[endorama]

I've reviewed the documentation for getting started. The only doubt I have is about APM Server with Fleet Server, where prerequisites (link) mention downloading the integration and some user permissions to install the assets. These prerequisites are shared with all documentation related to running Fleet Server, but don't look relevant to APM Server using the plugin. But are also quite safe. Per my understanding all other requirements are still relevant. All further steps are still relevant too.

Overall I would suggest not to proceed with further changes to address that I mention before, as those steps would still be needed for other interactions with Fleet Server.

[endorama]

Moving forward with my proposal as no objection has been done.