Update version in README to 2.18.0 (#15968)

Signed-off-by: Craig Perkins <craig5008@gmail.com>

README.md

@@ -7,7 +7,7 @@
 [![Security Vulnerabilities](https://img.shields.io/github/issues/opensearch-project/OpenSearch/security%20vulnerability?labelColor=red)](https://github.com/opensearch-project/OpenSearch/issues?q=is%3Aissue+is%3Aopen+label%3A"security%20vulnerability")
 [![Open Issues](https://img.shields.io/github/issues/opensearch-project/OpenSearch)](https://github.com/opensearch-project/OpenSearch/issues)
 [![Open Pull Requests](https://img.shields.io/github/issues-pr/opensearch-project/OpenSearch)](https://github.com/opensearch-project/OpenSearch/pulls)
-[![2.17.0 Open Issues](https://img.shields.io/github/issues/opensearch-project/OpenSearch/v2.17.0)](https://github.com/opensearch-project/OpenSearch/issues?q=is%3Aissue+is%3Aopen+label%3A"v2.17.0")
+[![2.18.0 Open Issues](https://img.shields.io/github/issues/opensearch-project/OpenSearch/v2.18.0)](https://github.com/opensearch-project/OpenSearch/issues?q=is%3Aissue+is%3Aopen+label%3A"v2.18.0")
 [![3.0.0 Open Issues](https://img.shields.io/github/issues/opensearch-project/OpenSearch/v3.0.0)](https://github.com/opensearch-project/OpenSearch/issues?q=is%3Aissue+is%3Aopen+label%3A"v3.0.0")
 [![GHA gradle check](https://github.com/opensearch-project/OpenSearch/actions/workflows/gradle-check.yml/badge.svg)](https://github.com/opensearch-project/OpenSearch/actions/workflows/gradle-check.yml)
 [![GHA validate pull request](https://github.com/opensearch-project/OpenSearch/actions/workflows/wrapper.yml/badge.svg)](https://github.com/opensearch-project/OpenSearch/actions/workflows/wrapper.yml)