Added release notes for v2.18 (opensearch-project#1282)

* Added release notes for v2.18

Signed-off-by: Kshitij Tandon <tandonks@amazon.com>

* Adding another recent commit in the release notes

Signed-off-by: Kshitij Tandon <tandonks@amazon.com>

---------

Signed-off-by: Kshitij Tandon <tandonks@amazon.com>

release-notes/opensearch-index-management.release-notes-2.18.0.0.md

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+## Version 2.18.0.0 2024-10-28
+
+Compatible with OpenSearch 2.18.0
+
+### Enhancements
+
+* Allowing non-rollup and rollup indices to be searched together ([#1268](https://github.com/opensearch-project/index-management/pull/1268))
+
+### Bug fixes
+* Fixing snapshot bug ([#1257](https://github.com/opensearch-project/index-management/pull/1257))
+
+### Maintenance
+* Increment version to 2.18.0-SNAPSHOT ([#1241](https://github.com/opensearch-project/index-management/pull/1241))
+* upgrade upload-artifact to version 3 ([#1252](https://github.com/opensearch-project/index-management/pull/1252))
+* bump bwc version after 2.17 release ([#1259](https://github.com/opensearch-project/index-management/pull/1259))
+* Move non-active maintainer to emeritus ([#1263](https://github.com/opensearch-project/index-management/pull/1263))
+* Update CI check for integ-test-with-security to run all integ tests with security ([#1243](https://github.com/opensearch-project/index-management/pull/1243))
+* Remove 2 instances wildcard imports ([#1251](https://github.com/opensearch-project/index-management/pull/1251))
+* Updating baseline JDK version to JDK-21 ([#1276](https://github.com/opensearch-project/index-management/pull/1276))