[AUTO] Add release notes for 3.9.0

[opensearch-ci-bot]

Add release notes for 3.9.0

Borderline Calls

• [AUTO] Add release notes for 3.8.0 #1748: Excluded — release notes addition for a previous version (3.8.0), not a change in this release.
• [AUTO] Increment version to 3.9.0-SNAPSHOT #1785: Excluded — automated version increment commit.
• Onboard new backport-pr re-usable github workflow (security-analytics) #1740: Excluded — only changes GitHub workflow configuration under .github/ (backport workflow onboarding).
• Onboards security-analytics plugin to centralized resource authz #1735: Placed in Features — introduces a net-new capability (centralized resource authorization) rather than enhancing an existing feature.
• Register detector and correlation-rule indices as system indices #1749: Placed in Features — registers indices as system indices, which is a new architectural capability required for the resource-sharing framework. Could be Infrastructure or Maintenance, but it introduces new runtime behavior (system index registration and stashed context changes).
• Rename resource sharing feature flag to the non-experimental key #1815: Placed in Maintenance — renames feature flag settings to match upstream graduation. Could be Infrastructure since it touches build/test configuration, but the setting rename has user-facing implications.
• Switch from snakeyaml to snakeyaml-engine for YAML processing #1812: Placed in Maintenance — dependency swap from snakeyaml to snakeyaml-engine. Could be Infrastructure, but dependency management fits Maintenance better.
• Fix codecoverage upload action #1810: Placed in Infrastructure — fixes CI code coverage upload, which is a CI/infrastructure concern.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.45%. Comparing base (9aa1f0c) to head (4f44ae8).

❌ Your project check has failed because the head coverage (68.45%) is below the target coverage (75.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1816      +/-   ##
============================================
- Coverage     68.58%   68.45%   -0.14%     
+ Complexity     3750     3749       -1     
============================================
  Files           417      417              
  Lines         20888    20888              
  Branches       2189     2189              
============================================
- Hits          14327    14299      -28     
- Misses         5461     5491      +30     
+ Partials       1100     1098       -2     

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