[AUTO] Add release notes for 3.8.0

[opensearch-ci-bot]

Add release notes for 3.8.0

Borderline Calls

• Use PPL to fetch sample data in PPLTool #752: Placed in Refactoring — could also be Enhancements since it unifies the sample retrieval path with the existing PPL execution approach. However, the core functionality (fetching a sample document for the prompt) remains unchanged, and the PR primarily restructures internal implementation, so Refactoring was chosen.
• Onboard code diff analyzer/reviewer and issue dedupe workflows (skills) #764: Placed in Enhancements due to the "enhancement" label — could also be Infrastructure since it onboards CI workflows.
• Onboard new backport-pr re-usable github workflow (skills) #759: Placed in Enhancements due to the "enhancement" label — could also be Infrastructure since it replaces a backport workflow.
• Sync CODEOWNERS with maintainer list #755: Placed in Maintenance — could also be Documentation since CODEOWNERS is a metadata/governance file, but routine upkeep of ownership files fits Maintenance better.
• Update actions/setup-java action to v5 #736: Placed in Infrastructure — borderline with Maintenance as it's a dependency/action version update, but since it directly modifies CI pipeline configuration, Infrastructure was chosen.

[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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.37%. Comparing base (2b76e3c) to head (05b6326).
⚠️ Report is 136 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #772      +/-   ##
============================================
- Coverage     81.78%   74.37%   -7.42%     
- Complexity      193      724     +531     
============================================
  Files            11       29      +18     
  Lines           961     3707    +2746     
  Branches        137      525     +388     
============================================
+ Hits            786     2757    +1971     
- Misses          121      750     +629     
- Partials         54      200     +146     

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