Document managed PRW support on ECH

[zmoog]

Summary

• mark the Managed Prometheus Remote Write endpoint as generally available on Elastic Cloud Hosted
• update recommendations, prerequisites, and endpoint discovery steps to include ECH deployments
• remove statements that ECH support is planned and that the endpoint is Serverless-only

Validation

• git diff --check
• local commit hooks
• full docs build and Vale validation run in CI

[github-actions]

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[github-actions]

🔍 Preview links for changed docs

• docs/reference/managed-inputs/prometheus-remote-write.md

[github-actions]

✅ Elastic Docs Style Checker (Vale)

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[zmoog]

@gizas, I would love your validation before submitting it to the docs team for the final review! 🙇

[florent-leborgne]

The changes look good to me.
For ECH, is there a minimum stack version to use? I left an inline suggestion with where to indicate it if necessary.

[florent-leborgne]

nice frontmatter updates 🚀

[florent-leborgne]

If there is a minimum stack version required for the ECH deployment, I'd propose something like this. Ignore if there is no specific version required:

Suggested change

	- An {{serverless-full}} Observability or Security project, or an {{ech}} deployment.
	- A compatible Elastic environment:
	- An {{serverless-full}} Observability or Security project
	- {applies_to}`stack: ga 9.6+` An {{ech}} deployment

For example if the min version is 9.6 ^^