[Website]: Cover limitation of Private Link for Managed Inputs on Serverless

### Before you submit

- [x] This issue is about a documentation page, flow, or piece of content.

### Type of issue

Missing information

### What documentation page or section is affected

https://www.elastic.co/docs/reference/opentelemetry/managed-inputs/authentication-delivery-and-failure-handling

### What happened?

We correctly document ECH limitation of ip filtering & privatelink. We might add a "planned" tag if we're working on it?

For Serverless, the `ingest` endpoint / Managed Input supports privatelink for AWS and Azure - GCP is pending (as per https://www.elastic.co/docs/deploy-manage/security/private-connectivity)

FYI @perk / @jlind23 / @raultorrecilla

### Additional info

_No response_

Open the affected authentication, delivery, and failure-handling page and compare its Private Link coverage with the private connectivity page. Document that Serverless Managed Inputs support Private Link for AWS and Azure while GCP is pending, and clarify whether a planned tag is needed.

Written by the indexing model from the issue text.