[AUTO] Add release notes for 3.8.0

[opensearch-ci-bot]

Add release notes for 3.8.0

Borderline Calls

• Onboard new backport-pr re-usable github workflow (neural-search) #1881: Placed in Infrastructure — could also be Maintenance since it replaces an existing workflow, but chose Infrastructure because it modifies CI/CD pipeline configuration.
• Skip two phase rescore for sort query #1898: Placed in Enhancements — could be Bug Fixes since it prevents incorrect behavior with sort queries, but the change is an optimization skip rather than fixing broken functionality.
• Improve hybrid query filter validation error message #1870: Placed in Enhancements based on the "enhancement" label — could also be Bug Fixes since it addresses a confusing error message.

[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 83.43%. Comparing base (094d089) to head (90a74bf).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1905      +/-   ##
============================================
- Coverage     83.45%   83.43%   -0.02%     
+ Complexity     3898     3897       -1     
============================================
  Files           291      291              
  Lines         13844    13844              
  Branches       2304     2304              
============================================
- Hits          11553    11551       -2     
- Misses         1456     1458       +2     
  Partials        835      835              

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