increment version to 2.16.0 (#1197)

Signed-off-by: Joanne Wang <[email protected]>
(cherry picked from commit 0d7bbe2)

build.gradle

@@ -7,7 +7,7 @@ import org.opensearch.gradle.test.RestIntegTestTask
 
 buildscript {
     ext {
-        opensearch_version = System.getProperty("opensearch.version", "2.15.0-SNAPSHOT")
+        opensearch_version = System.getProperty("opensearch.version", "2.16.0-SNAPSHOT")
         isSnapshot = "true" == System.getProperty("build.snapshot", "true")
         buildVersionQualifier = System.getProperty("build.version_qualifier", "")
         version_tokens = opensearch_version.tokenize('-')