[AUTO] Add release notes for 3.8.0

[opensearch-ci-bot]

Add release notes for 3.8.0

Borderline Calls

• Implement DocRequest on notification config and alerting request classes #980: Placed in Features — this introduces a new capability (automatic transport-layer authorization for resource access) rather than merely enhancing an existing feature. Could be argued as Enhancement since it builds on existing request classes, but the net-new authorization integration justifies Features.
• Update opensearch-build workflow references from SHA to @main #971: Placed in Infrastructure — could also be Maintenance since it's updating workflow references, but it modifies CI/CD configuration which aligns more with Infrastructure.

[github-actions]

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🧪 No relevant tests

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