[Backport 1.3] Release notes for 1.3.15

[kotwanikunal]

Description

Release notes for v1.3.15
Also cleans up the changelog

Related Issues

N/A

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Failing checks are inspected and point to the corresponding known issue(s) (See: Troubleshooting Failing Builds)
• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)
• Public documentation issue/PR created

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[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/34313/
• CommitID: 04af21e

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.63%. Comparing base (a994fbd) to head (04af21e).

Additional details and impacted files

@@             Coverage Diff              @@
##                1.3   #12511      +/-   ##
============================================
- Coverage     77.64%   77.63%   -0.01%     
- Complexity    58761    58816      +55     
============================================
  Files          4223     4223              
  Lines        253440   253440              
  Branches      38701    38701              
============================================
- Hits         196788   196768      -20     
- Misses        40623    40682      +59     
+ Partials      16029    15990      -39     

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