Increment version to 1.3.20-SNAPSHOT (#1286)

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

Author: opensearch-trigger-bot[bot]

build.gradle

@@ -17,7 +17,7 @@ buildscript {
     ext {
         opensearch_group = "org.opensearch"
         isSnapshot = "true" == System.getProperty("build.snapshot", "true")
-        opensearch_version = System.getProperty("opensearch.version", "1.3.19-SNAPSHOT")
+        opensearch_version = System.getProperty("opensearch.version", "1.3.20-SNAPSHOT")
         if (isSnapshot) {
             plugin_version = opensearch_version.substring(0, opensearch_version.indexOf("-"))
         } else {