Getting a list of regions that support DocumentDB

### Describe the bug

List of supported regions for DocumentDB returned by `software.amazon.awssdk.regions.servicemetadata.DocdbServiceMetadata#regions` does not include eu-south-1, although this service is available in this region.
List of supported regions for DocumentDB returned by `DocDbClient.serviceMetadata().regions()` returns a superset of regions that document db is supported in as it resolves to RDS regions and some of them do not have support for DocDB.

### Regression Issue

- [ ] Select this option if this issue appears to be a regression.

### Expected Behavior

I expect both/either of these methods to return the list of all regions that support DocDB.

### Current Behavior

List of supported regions for DocDB does not include eu-south-1 region if you get them using: `software.amazon.awssdk.regions.servicemetadata.DocdbServiceMetadata#regions`.
List of supported regions for DocDB contains regions that have support for RDS, but not for DocDB if you get them using `DocDbClient.serviceMetadata().regions()`.

### Reproduction Steps

Call `software.amazon.awssdk.regions.servicemetadata.DocdbServiceMetadata#regions` method to get list of supported regions for DocDB service.
Call `DocDbClient.serviceMetadata().regions()` method to get list of supported regions for DocDB service

### Possible Solution

_No response_

### Additional Information/Context

This is a follow-up to: https://github.com/aws/aws-sdk-java-v2/issues/5585

### AWS Java SDK version used

2.29.43

### JDK version used

openjdk 21.0.5 2024-10-15

### Operating System and version

macOS 15.1.1