Updates changelog structure to adhere to opensearch release-notes guideline

[DarshitChanpura]

Updates changelog layout to be in-line with the guideline stated here:
https://github.com/opensearch-project/opensearch-plugins/blob/main/RELEASE_NOTES.md

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.96%. Comparing base (5ac567e) to head (6972fc5).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2271   +/-   ##
=======================================
  Coverage   71.96%   71.96%           
=======================================
  Files          98       98           
  Lines        2679     2679           
  Branches      417      409    -8     
=======================================
  Hits         1928     1928           
  Misses        637      637           
  Partials      114      114           

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