chore(elasticsearch): remove elasticsearch legacy reference (backport #23943) (#23949)

This is an automatic backport of pull request #23943 done by [Mergify](https://mergify.com).


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Author: mergify[bot]

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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