Increment version to 2.19.0-SNAPSHOT (opensearch-project#3169)

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

@@ -9,7 +9,7 @@ buildscript {
     ext {
         opensearch_group = "org.opensearch"
         isSnapshot = "true" == System.getProperty("build.snapshot", "true")
-        opensearch_version = System.getProperty("opensearch.version", "2.18.0-SNAPSHOT")
+        opensearch_version = System.getProperty("opensearch.version", "2.19.0-SNAPSHOT")
         buildVersionQualifier = System.getProperty("build.version_qualifier", "")
 
         // 2.0.0-rc1-SNAPSHOT -> 2.0.0.0-rc1-SNAPSHOT