[Backport 2.x] Add release notes for 1.3.15

[opensearch-trigger-bot]

Backport 5c4590d from #2536

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.47%. Comparing base (2d165e4) to head (af5560e).

Additional details and impacted files

@@            Coverage Diff            @@
##                2.x    #2537   +/-   ##
=========================================
  Coverage     95.47%   95.47%           
  Complexity     5064     5064           
=========================================
  Files           484      484           
  Lines         14183    14183           
  Branches        947      947           
=========================================
  Hits          13541    13541           
  Misses          621      621           
  Partials         21       21           

Flag	Coverage Δ
sql-engine	95.47% <ø> (ø)

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