[AUTO] Add release notes for 2.19.6

[opensearch-ci-bot]

Add release notes for 2.19.6

Borderline Calls

• Use ObjectInputFilter for deserialization #5469: Placed in Bug Fixes based on the "bug" label. Could also be considered Infrastructure or Enhancements since it adds a security hardening mechanism (allowlist-based deserialization filtering) rather than fixing a user-facing defect.
• [Backport 2.19] Add CI mirror to avoid Maven Central throttling #5591: Placed in Infrastructure — could also be Maintenance since it's a build configuration change, but chose Infrastructure because it modifies the CI/build pipeline.
• Pin GitHub Actions to commit SHAs #5573: Placed in Infrastructure — could also be Maintenance, but chose Infrastructure because it directly modifies CI workflow files for security hardening.

[github-actions]

PR Reviewer Guide 🔍

(Review updated until commit dd8135d)

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

[github-actions]

Persistent review updated to latest commit dd8135d

[peterzhuamazon]

LGTM