(app-staging-synthesizer): Utilize Account Regional namespace suffix for default StagingBucket

### Describe the feature

As described [here](https://aws.amazon.com/blogs/aws/introducing-account-regional-namespaces-for-amazon-s3-general-purpose-buckets/), S3 now has an account regional namespace suffix. For the purposes of CDK, I think it just requires appending `-an` to the default StagingBucket's name (and handling the breaking change - which is more work).

https://github.com/aws/aws-cdk/blob/6903378baadb4fc0b14ce75b7ceb90ec14adf208/packages/%40aws-cdk/app-staging-synthesizer-alpha/lib/default-staging-stack.ts#L380

### Use Case

I want to protect against bucket squatting for CDK buckets.

### Proposed Solution

_No response_

### Other Information

For my own personal accounts, I can customize the staging bucket name so no change is needed, but making this the default for future accounts could be a good idea.

### Acknowledgements

- [x] I may be able to implement this feature request
- [x] This feature might incur a breaking change

### AWS CDK Library version (aws-cdk-lib)

2.243.0

### AWS CDK CLI version

2.243.0

### Environment details (OS name and version, etc.)

N/A

Start with packages/@aws-cdk/app-staging-synthesizer-alpha/lib/default-staging-stack.ts at the referenced default StagingBucket definition. Read the surrounding staging synthesizer behavior and review the account regional namespace suffix described in the linked AWS announcement. Done means the default bucket naming uses the suffix and the associated breaking-change handling is addressed.

Written by the indexing model from the issue text.