[AUTO] Add release notes for 3.8.0

[opensearch-ci-bot]

Add release notes for 3.8.0

Borderline Calls

• Introduces BulkVectorScorer for ExactSearch #3361: Placed in Enhancements — labeled as both "Refactoring" and "enhancement". Chose Enhancements because the PR introduces a new class (BulkVectorScorer) that improves the exact-search architecture, though it also refactors existing code. Could reasonably be placed in Refactoring.
• Check to see if Lucene's search budget has exhausted when deciding to exact search #3354: Placed in Enhancements — could also be a Bug Fix since it corrects behavior where MOS didn't respect Lucene's budget exhaustion. Chose Enhancements because it improves recall behavior rather than fixing a crash or incorrect result.
• Parameterize tests based on compression level for 32x defaults #3416: Placed in Infrastructure — could be Maintenance since it's test preparation for a future default change. Chose Infrastructure because it adds test framework classes and parameterization.

[github-actions]

PR Reviewer Guide 🔍

Here are some key observations to aid the review process:

🧪 No relevant tests

🔒 No security concerns identified

✅ No TODO sections

🔀 No multiple PR themes

⚡ No major issues detected