Amazon Elasticsearch Service is being rebranded to Amazon OpenSearch Service. All customer-facing references to Elasticsearch within AWS CDK must be replaced with their OpenSearch equivalent, including documentation and API. To avoid breaking existing customers, all API changes will be implemented as a deprecation of the old API and introduction of a new, rebranded API.

Concretely, this change consists of the following:

- add a disclaimer at the top of the @aws-cdk/aws-elasticsearch module README about the rebranding
- add migration instructions to the @aws-cdk/aws-elasticsearch module README
- introduce the @aws-cdk/aws-opensearch module
  - copy all files from the Elasticsearch module
  - uses new AWS::OpenSearch::Domain resource. CFN guarantees a no-replacement update when a resource moves from AWS::Elasticsearch::Domain to AWS::OpenSearch::Domain, meaning customers should be able to move from the old CDK module to the new one with no downtime.
  - rename DomainProps.cognitoKibanaAuth -> DomainProps.cognitoDashboardsAuth
  - replaces ElasticsearchVersion with OpenSearchVersion
    - ElasticsearchVersion.V{major}_{minor} -> OpenSearchVersion.ELASTICSEARCH_{major}_{minor}
    - ElasticsearchVersion.of(version: string) -> OpenSearchVersion.elasticsearch(version: string)
    - introduces OpenSearchVersion.OPENSEARCH_1_0 and OpenSearchVersion.openSearch(version: string)
  - replace .elasticsearch instance suffix with .search
  - rename logical ID and type of a custom resource (Custom::ElasticsearchAccessPolicy -> Custom::OpenSearchAccessPolicy) that we generate to update the domain's access policy. This will cause a resource replacement, which will avoid a CloudFormation error (Update of resource type is not permitted) for stacks that switch modules

Closes #16467