[Backport 2.1] Adds v2.1.0 release notes

[opensearch-trigger-bot]

Backport d5611d5 from #1809

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (2.1@e1380f1). Click here to learn what that means.
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❗ Current head d5611d5 differs from pull request most recent head 50b849d. Consider uploading reports for the commit 50b849d to get more accurate results

@@          Coverage Diff           @@
##             2.1    #1853   +/-   ##
======================================
  Coverage       ?   67.50%           
======================================
  Files          ?     3074           
  Lines          ?    59127           
  Branches       ?     8988           
======================================
  Hits           ?    39915           
  Misses         ?    17028           
  Partials       ?     2184           

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