AWS::SecurityHub::FindingAggregator - [Coverage] - Missing NO_REGIONS linking mode

### Name of the resource

Other

### Resource name

AWS::SecurityHub::FindingAggregator

### Description

The `RegionLinkingMode` property is missing the ability to specify `NO_REGIONS` as the aggregation type. Because centralized configuration requires an aggregator to set up this means it is impossible to enable centralized configuration in an organization that only uses a single region and has guardrails enabled to prevent use of unused regions.

See [RegionLinkingMode](https://docs.aws.amazon.com/securityhub/1.0/APIReference/API_CreateFindingAggregator.html#API_CreateFindingAggregator_RequestSyntax)

### Other Details

_No response_

Start with the linked AWS CreateFindingAggregator API documentation and inspect how AWS::SecurityHub::FindingAggregator represents RegionLinkingMode. The work is done when the resource supports NO_REGIONS as an aggregation type and its coverage reflects the documented option.

Written by the indexing model from the issue text.