Update reference in README from 2.4.0 to 2.18.0 (#4806)

Signed-off-by: Craig Perkins <cwperx@amazon.com>

README.md

@@ -1,5 +1,5 @@
 [![CI](https://github.com/opensearch-project/security/workflows/CI/badge.svg?branch=main)](https://github.com/opensearch-project/security/actions) [![](https://img.shields.io/github/issues/opensearch-project/security/untriaged?labelColor=red)](https://github.com/opensearch-project/security/issues?q=is%3Aissue+is%3Aopen+label%3A"untriaged") [![](https://img.shields.io/github/issues/opensearch-project/security/security%20vulnerability?labelColor=red)](https://github.com/opensearch-project/security/issues?q=is%3Aissue+is%3Aopen+label%3A"security%20vulnerability") [![](https://img.shields.io/github/issues/opensearch-project/security)](https://github.com/opensearch-project/security/issues) [![](https://img.shields.io/github/issues-pr/opensearch-project/security)](https://github.com/opensearch-project/security/pulls)
-[![](https://img.shields.io/codecov/c/gh/opensearch-project/security)](https://app.codecov.io/gh/opensearch-project/security) [![](https://img.shields.io/github/issues/opensearch-project/security/v2.4.0)](https://github.com/opensearch-project/security/issues?q=is%3Aissue+is%3Aopen+label%3A"v2.4.0") [![](https://img.shields.io/github/issues/opensearch-project/security/v3.0.0)](https://github.com/opensearch-project/security/issues?q=is%3Aissue+is%3Aopen+label%3A"v3.0.0")
+[![](https://img.shields.io/codecov/c/gh/opensearch-project/security)](https://app.codecov.io/gh/opensearch-project/security) [![](https://img.shields.io/github/issues/opensearch-project/security/v2.18.0)](https://github.com/opensearch-project/security/issues?q=is%3Aissue+is%3Aopen+label%3A"v2.18.0") [![](https://img.shields.io/github/issues/opensearch-project/security/v3.0.0)](https://github.com/opensearch-project/security/issues?q=is%3Aissue+is%3Aopen+label%3A"v3.0.0")
 [![Slack](https://img.shields.io/badge/Slack-4A154B?&logo=slack&logoColor=white)](https://opensearch.slack.com/archives/C051Y637FKK)