chore(elasticsearch): remove elasticsearch legacy reference (#23943)

Elastic has followed up with a request that we remove "legacy Elasticsearch" references from our docs for legal reasons.

They supplied these two links to remove the string from: https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_elasticsearch-readme.html and https://docs.aws.amazon.com/cdk/api/v1/docs/aws-elasticsearch-readme.html

Internal reference: V825034754.

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packages/@aws-cdk/aws-elasticsearch/README.md

@@ -1,4 +1,4 @@
-# Amazon OpenSearch Service (legacy Elasticsearch) Construct Library
+# Amazon OpenSearch Service Construct Library
 <!--BEGIN STABILITY BANNER-->
 
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