Increment version to 2.6.0-SNAPSHOT (#671)

Signed-off-by: opensearch-ci-bot <[email protected]>

Signed-off-by: opensearch-ci-bot <[email protected]>
Co-authored-by: opensearch-ci-bot <[email protected]>

Author: opensearch-trigger-bot[bot]

build.gradle

@@ -9,7 +9,7 @@ buildscript {
     ext {
         opensearch_group = "org.opensearch"
         isSnapshot = "true" == System.getProperty("build.snapshot", "true")
-        opensearch_version = System.getProperty("opensearch.version", "2.5.0-SNAPSHOT")
+        opensearch_version = System.getProperty("opensearch.version", "2.6.0-SNAPSHOT")
         buildVersionQualifier = System.getProperty("build.version_qualifier", "")
 
         // 2.0.0-rc1-SNAPSHOT -> 2.0.0.0-rc1-SNAPSHOT