Added 2.18.0 release notes.

[AWSHurneyt]

Description

Added 2.18.0 release notes.

Related Issues

https://github.com/opensearch-project/alerting/issues/1654

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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