Increment version to 1.3.8-SNAPSHOT (opensearch-project#338)

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

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>

build.gradle

@@ -6,7 +6,7 @@
 buildscript {
     ext {
         opensearch_group = "org.opensearch"
-        opensearch_version = System.getProperty("opensearch.version", "1.3.7-SNAPSHOT")
+        opensearch_version = System.getProperty("opensearch.version", "1.3.8-SNAPSHOT")
         kotlin_version = System.getProperty("kotlin.version", "1.6.0")
     }