Increment version to 2.15.0-SNAPSHOT (#918)

Signed-off-by: opensearch-ci-bot <opensearch-infra@amazon.com>

Author: opensearch-trigger-bot[bot]

notifications/build.gradle

@@ -6,7 +6,7 @@
 
 buildscript {
     ext {
-        opensearch_version = System.getProperty("opensearch.version", "2.14.0-SNAPSHOT")
+        opensearch_version = System.getProperty("opensearch.version", "2.15.0-SNAPSHOT")
         isSnapshot = "true" == System.getProperty("build.snapshot", "true")
         buildVersionQualifier = System.getProperty("build.version_qualifier", "")
         // 2.3.0-SNAPSHOT -> 2.3.0.0-SNAPSHOT