[Internal]: serverless changelog Sept 29

### Description

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### Resources

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### Which documentation set does this change impact?

Elastic On-Prem and Cloud (all)

### Feature differences

-

### What release is this request related to?

N/A

### Serverless release

September 29, 2025

### Collaboration model

The documentation team

### Point of contact.

**Main contact:** @

**Stakeholders:**
