Increment version to 2.20.0-SNAPSHOT (#238)

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

@@ -8,7 +8,7 @@ import java.util.stream.Collectors
 
 buildscript {
   ext {
-    opensearch_version = System.getProperty("opensearch.version", "2.19.0-SNAPSHOT")
+    opensearch_version = System.getProperty("opensearch.version", "2.20.0-SNAPSHOT")
     isSnapshot = "true" == System.getProperty("build.snapshot", "true")
     buildVersionQualifier = System.getProperty("build.version_qualifier", "")
     version_tokens = opensearch_version.tokenize('-')