[REQUEST]: Document known issue about agentless Fleet Server connectivity

[jen-huang]

Description

It is possible to break connectivity to Fleet Server for agentless integrations on Cloud in a few ways, by either editing the default Fleet Server host URL value, or by setting a different Fleet Server host as the default.

This is fixed for ECH from 9.1.6 onwards, and already fixed on serverless (technically from 9.1.0 onwards).

Resources

https://github.com/elastic/ingest-dev/issues/5924#issuecomment-3144569962
elastic/kibana#218905
elastic/kibana#225699
elastic/kibana#225188

Collaboration

The documentation team will investigate the issue and create the initial content.

Point of contact.

Main contact: @jen-huang

Stakeholders: