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.

Details

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.