[AUTO] Add release notes for 3.9.0

[opensearch-ci-bot]

Add release notes for 3.9.0

Borderline Calls

• Onboards notifications plugin to centralized resource authz #1237: Placed in Features — this onboards the notifications plugin to centralized resource authorization, which is a net new capability (resource-access-control for notification_config). Could be considered an Enhancement since the plugin already existed, but the resource sharing integration is new functionality.
• Fix Notifications CI dependency failures #1268: Placed in Infrastructure — this fixes CI build failures by aligning dependencies and adding missing classpath entries. Could be Maintenance since it involves dependency management, but the primary purpose is restoring CI functionality.
• Fix code-coverage action #1269: Placed in Infrastructure — straightforward CI fix.
• Rename resource sharing feature flag to the non-experimental key #1271: Placed in Maintenance — this only renames setting keys in integration test configuration to match an upstream rename. Could be Infrastructure since it only affects test/build config, but the setting rename reflects a broader platform change that operators should be aware of.
• Onboard code diff analyzer/reviewer and issue dedupe workflows (notifications) #1219: Excluded — only onboards GitHub workflow/automation config (code diff analyzer/reviewer and issue dedupe workflows), which falls under .github/ workflow changes.
• Fix SES/SNS NoClassDefFoundError from Jackson 3 / HttpClient5 migration #1257: Placed in Bug Fixes — restores dependencies that were inadvertently removed during a migration, causing NoClassDefFoundError in production. Could be Infrastructure since it modifies build.gradle, but the root cause is a production runtime failure.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.73%. Comparing base (de3450d) to head (3aaa299).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1272   +/-   ##
=========================================
  Coverage     70.73%   70.73%           
  Complexity      308      308           
=========================================
  Files            45       45           
  Lines          1941     1941           
  Branches        145      145           
=========================================
  Hits           1373     1373           
  Misses          475      475           
  Partials         93       93           

Flag	Coverage Δ
opensearch-notifications	70.73% <ø> (ø)

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