Increment version to 2.3.1-SNAPSHOT (#250)

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

Author: opensearch-trigger-bot[bot]

build.gradle

@@ -6,7 +6,7 @@
 buildscript {
     ext {
         opensearch_group = "org.opensearch"
-        opensearch_version = System.getProperty("opensearch.version", "2.3.0-SNAPSHOT")
+        opensearch_version = System.getProperty("opensearch.version", "2.3.1-SNAPSHOT")
         isSnapshot = "true" == System.getProperty("build.snapshot", "true")
         buildVersionQualifier = System.getProperty("build.version_qualifier", "")
         kotlin_version = System.getProperty("kotlin.version", "1.6.10")