[apm] Improve instructions for monitoring APM on premise

[colleenmcginnis]

Related to elastic/observability-docs#4726

Ports changes from elastic/observability-docs#4749 to 9.0 docs. Here's some context from the initial PR:

@lucabelluccini shared a few ways we could improve the instructions for monitoring APM on premise:

• Warn users that if they have already enrolled Elastic Agent, following these instructions will likely cause them to lose the state of the agent.
• Suggest that users who have already enrolled Elastic Agent, use Fleet UI to change the agent's HTTP monitoring endpoint.
• Remove username and password from the sample modules.d/beat-xpack.yml.

[bmorelli25]

@lucabelluccini can you take a look at this PR please

[colleenmcginnis]

👋 Also tagging @raultorrecilla for review since Silvia suggested tagging him in elastic/observability-docs#4749.