[AUTO] Add release notes for 3.8.0

[mengweieric]

Add release notes for 3.8.0

Draft / preview. OpenSearch 3.8 has not been branch-cut yet (latest tag is 3.7.0.0). These notes are generated from every changelog-eligible PR merged to main since the 3.7.0 release-notes commit (#5472), following .github/draft-release-notes-config.yml (label→category mapping, sort-by: merged_at ascending). Expect the list to grow until 3.8 is actually cut; the official opensearch-ci-bot [AUTO] PR will supersede this at release time.

Borderline Calls

• PPL rest command excluded — added in [Feature] Add PPL rest command #5599 but fully reverted by Revert "[Feature] Add PPL rest command" #5635, so it nets to zero for 3.8 and is intentionally omitted (both PRs dropped rather than listed).
• [Release 3.7.0] Patch release note 3.7 #5494 ("Patch release note 3.7") excluded — it only edits the 3.7.0.0 notes file (3.7 housekeeping), not a 3.8 change.
• Analytics-engine IT parity/stabilization cluster — ~24 PRs (e.g. "Bring Calcite…IT to parity on the analytics-engine route", "Stabilize PPL ITs on the analytics-engine route (…)") are split across Enhancements, Bug Fixes, and Infrastructure per their PR labels. They dominate this release's volume; maintainers may wish to condense them into a summary line at cut time.
• Anonymize xyseries command and mark it experimental in docs #5643 ("Anonymize xyseries command and mark it experimental in docs") placed in Enhancements per its enhancement label, though it is partly documentation.
• Reverts/version bumps (Increment version to 3.8.0-SNAPSHOT #5498 version→3.8.0-SNAPSHOT) placed in Maintenance.

Category counts: Features 6 · Enhancements 24 · Bug Fixes 27 · Infrastructure 24 · Maintenance 4 (total 85).

[github-actions]

PR Reviewer Guide 🔍

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🧪 No relevant tests

🔒 No security concerns identified

✅ No TODO sections

🔀 No multiple PR themes

⚡ No major issues detected