Increment version to 2.8.1-SNAPSHOT (#809)

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

Author: opensearch-trigger-bot[bot]

build.gradle

@@ -26,7 +26,7 @@ import java.util.concurrent.TimeUnit
 buildscript {
     ext {
         isSnapshot = "true" == System.getProperty("build.snapshot", "true")
-        opensearch_version = System.getProperty("opensearch.version", "2.8.0-SNAPSHOT")
+        opensearch_version = System.getProperty("opensearch.version", "2.8.1-SNAPSHOT")
         buildVersionQualifier = System.getProperty("build.version_qualifier", "")
         // 2.2.0-SNAPSHOT -> 2.2.0.0-SNAPSHOT
         version_tokens = opensearch_version.tokenize('-')