[Backport 2.1] Added 2.1 release notes.

[opensearch-trigger-bot]

Backport 9f3c393 from #485

[codecov-commenter]

Codecov Report

Merging #486 (d33d9aa) into 2.1 (6d3fb50) will decrease coverage by 0.14%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.1     #486      +/-   ##
============================================
- Coverage     76.89%   76.75%   -0.15%     
  Complexity      176      176              
============================================
  Files           166      166              
  Lines          8370     8370              
  Branches       1232     1232              
============================================
- Hits           6436     6424      -12     
- Misses         1329     1342      +13     
+ Partials        605      604       -1     

Impacted Files	Coverage Δ
...ing/model/destination/DestinationContextFactory.kt	75.00% <0.00%> (-3.58%)	⬇️
.../alerting/transport/TransportIndexMonitorAction.kt	68.18% <0.00%> (-3.15%)	⬇️
.../kotlin/org/opensearch/alerting/util/IndexUtils.kt	70.83% <0.00%> (-2.09%)	⬇️
...rch/alerting/opensearchapi/OpenSearchExtensions.kt	54.05% <0.00%> (-1.36%)	⬇️

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