feat(opensearchservice): add useLatestServiceSoftwareForBlueGreen support (#38499)

### Issue #38664 

Closes #38664 .

### Issue

Adds support for `UseLatestServiceSoftwareForBlueGreen` in the OpenSearch domain `SoftwareUpdateOptions`, aligning the L2 construct with the service API.

### Reason for this change

The OpenSearch Service API supports `UseLatestServiceSoftwareForBlueGreen` in `SoftwareUpdateOptions`, which allows domains to automatically use the latest available service software when a blue/green deployment is triggered. This property is already in the CloudFormation resource schema (`AWS::OpenSearchService::Domain`) but was missing from the CDK L2 construct.

### Description of changes

- Added `useLatestServiceSoftwareForBlueGreen` optional boolean prop to `DomainProps`
- Updated L1 wiring to pass the property through to `softwareUpdateOptions`
- Updated the `softwareUpdateOptions` condition to render when either `enableAutoSoftwareUpdate` or `useLatestServiceSoftwareForBlueGreen` is specified
- Updated README with usage example
- Added unit tests

### Description of how you validated changes

Unit tests covering:
- `useLatestServiceSoftwareForBlueGreen` set to `true`
- `useLatestServiceSoftwareForBlueGreen` set to `false`
- Both props used together
- Existing tests still pass (absent when neither prop specified)

### Checklist

- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md)
- [x] Is this a breaking change (changing existing behavior or interface)? No
- [x] Does this change require a documentation update? Yes (README updated)
- [x] Does this change require an update to the integration tests? Not required (no new CFN resource type)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: Swathi-Karanth