Increment version to 3.1.0-SNAPSHOT (opensearch-project#3588)

Signed-off-by: opensearch-ci-bot <opensearch-infra@amazon.com>
Co-authored-by: opensearch-ci-bot <opensearch-infra@amazon.com>
Co-authored-by: Peter Zhu <zhujiaxi@amazon.com>

Author: 3 people

build.gradle

@@ -6,7 +6,7 @@
 
 buildscript {
     ext {
-        opensearch_version = System.getProperty("opensearch.version", "3.0.0-beta1-SNAPSHOT")
+        opensearch_version = System.getProperty("opensearch.version", "3.1.0-SNAPSHOT")
         isSnapshot = "true" == System.getProperty("build.snapshot", "true")
         buildVersionQualifier = System.getProperty("build.version_qualifier", "")
         version_tokens = opensearch_version.tokenize('-')