[AUTO] Add release notes for 3.9.0

[opensearch-ci-bot]

Add release notes for 3.9.0

Borderline Calls

• Always use 1 shard for system indexes #1468: Placed in Infrastructure — this changes the default shard count for internal system indexes, which is an infrastructure/operational concern rather than a user-facing feature or bug fix. Could arguably be Maintenance.
• Rename resource sharing feature flag to the non-experimental key #1482: Placed in Maintenance — this only renames feature flag setting strings in build configuration to align with an upstream security plugin change. Borderline with exclusion under the "only change build configuration" rule, but the rename is required for functional compatibility with the new security plugin version, so it was kept. Also borderline with Infrastructure.
• Declare workflow_state as child resource of workflow #1455: The PR description and labels indicate "Bug fix" category. The commit fixes incorrect resource-sharing behavior for workflow_state documents, which aligns with Bug Fixes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.50%. Comparing base (38e541d) to head (a9300a7).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1485      +/-   ##
============================================
- Coverage     78.55%   78.50%   -0.05%     
  Complexity     1315     1315              
============================================
  Files           106      106              
  Lines          6048     6048              
  Branches        647      647              
============================================
- Hits           4751     4748       -3     
- Misses         1006     1010       +4     
+ Partials        291      290       -1     

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