[Lambda] [DocumentDB] Support Amazon DocumentDB 8.0.x as a Lambda event source

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**Tell us about your request**

Add Amazon DocumentDB 8.0.x to the engine versions accepted by Lambda's `CreateEventSourceMapping` (and `UpdateEventSourceMapping`) for `aws:docdb` event sources. Today only DocumentDB 4.0 and 5.0 are accepted.

**Which service(s) is this request for?**

Lambda/DocumentDB

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

We use DocumentDB change streams as a native trigger for Lambda (event-driven processing of document changes). Our clusters run 8.0.1, and creating the trigger fails at the API:

```
operation error Lambda: CreateEventSourceMapping,
https response error StatusCode: 400,
InvalidParameterValueException: Engine version 8.0.1 is not supported.
```

This looks like a create-time version allowlist rather than a functional limitation. If we create the mapping while the cluster is on 5.0.0 and then upgrade the cluster in place to 8.0.1, the existing trigger keeps working — events continue flowing to the function with no changes. So the 8.0 engine is already compatible with the change-stream / event-source-mapping mechanism at runtime; only `CreateEventSourceMapping` rejects 8.0.x up front.

Impact: we can't run clusters on the current engine version and use the fully managed native trigger at the same time. That blocks us from upgrading, or forces extra infrastructure.

**Are you currently working around this issue?**

Create the event source mapping on a 5.0.0 cluster, then upgrade the cluster in place to 8.0.1 - the trigger survives the upgrade. Unfortunately you will be unable to add new triggers in future, which is blocker for us.

**Additional context**

The supported-versions notes in "Process Amazon DocumentDB events with Lambda" (Lambda dev guide) and "Using AWS Lambda with change streams" (DocumentDB dev guide) currently list only 4.0 and 5.0 and don't mention 8.0, so the error is ambiguous with a misconfiguration. Until 8.0 is supported, explicitly calling it out in both docs pages (and ideally the error message) would help. Cluster type is instance-based / regional (not elastic or global), with change streams enabled.