[Backport 2.1] 2.1 release note

[opensearch-trigger-bot]

Backport b586ec8 from #405

[codecov-commenter]

Codecov Report

Merging #406 (9f57206) into 2.1 (be2b093) will increase coverage by 0.10%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.1     #406      +/-   ##
============================================
+ Coverage     75.58%   75.68%   +0.10%     
- Complexity     2457     2463       +6     
============================================
  Files           313      313              
  Lines         14426    14426              
  Branches       2230     2230              
============================================
+ Hits          10904    10919      +15     
+ Misses         2276     2265      -11     
+ Partials       1246     1242       -4     

Impacted Files	Coverage Δ
.../opensearch/indexmanagement/rollup/model/Rollup.kt	86.04% <0.00%> (+0.46%)	⬆️
...nt/indexstatemanagement/ManagedIndexCoordinator.kt	69.29% <0.00%> (+0.58%)	⬆️
...ensearch/indexmanagement/rollup/model/ISMRollup.kt	96.52% <0.00%> (+1.73%)	⬆️
...management/indexstatemanagement/MetadataService.kt	64.92% <0.00%> (+2.23%)	⬆️
...anagement/indexstatemanagement/model/Transition.kt	65.75% <0.00%> (+4.10%)	⬆️
...nt/indexstatemanagement/model/destination/Slack.kt	77.77% <0.00%> (+22.22%)	⬆️

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