Increment version to 2.20.0-SNAPSHOT (#1162)

Signed-off-by: opensearch-ci-bot <[email protected]>
Co-authored-by: opensearch-ci-bot <[email protected]>

Author: opensearch-trigger-bot[bot]

build.gradle

@@ -14,7 +14,7 @@ import java.util.concurrent.Callable
 
 buildscript {
     ext {
-        opensearch_version = System.getProperty("opensearch.version", "2.19.0-SNAPSHOT")
+        opensearch_version = System.getProperty("opensearch.version", "2.20.0-SNAPSHOT")
         buildVersionQualifier = System.getProperty("build.version_qualifier", "")
         isSnapshot = "true" == System.getProperty("build.snapshot", "true")
         version_tokens = opensearch_version.tokenize('-')