[Backport 2.x] Add Release Notes for 2.18.0.0 (#2227)

[naveentatikonda]

Description

Backport https://github.com/opensearch-project/k-NN/pull/2227 (cherry picked from commit 8f2d9114df33d232a9b749b8863922219cf56afd)

Check List

• [ ] New functionality includes testing.
• [ ] New functionality has been documented.
• [ ] API changes companion pull request created.
• [x] Commits are signed per the DCO using --signoff.
• [ ] Public documentation issue/PR created.

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