Increment version to 2.13.0-SNAPSHOT (opensearch-project#591)

Signed-off-by: opensearch-ci-bot <opensearch-infra@amazon.com>
Co-authored-by: opensearch-ci-bot <opensearch-infra@amazon.com>
Signed-off-by: AWSHurneyt <hurneyt@amazon.com>

Author: 2 people

build.gradle

@@ -6,7 +6,7 @@
 buildscript {
     ext {
         opensearch_group = "org.opensearch"
-        opensearch_version = System.getProperty("opensearch.version", "2.12.0-SNAPSHOT")
+        opensearch_version = System.getProperty("opensearch.version", "2.13.0-SNAPSHOT")
         isSnapshot = "true" == System.getProperty("build.snapshot", "true")
         buildVersionQualifier = System.getProperty("build.version_qualifier", "")
         kotlin_version = System.getProperty("kotlin.version", "1.8.21")