Upgrade 2.x branch to 2.11.1 (#209)

I'm going to cut a 2.11 branch from this commit on 2.x.

After I cut the 2.11 branch, I'm going to upgrade 2.x to 2.12.0.

Signed-off-by: Michael Froh <froh@amazon.com>

build.gradle

@@ -1,6 +1,6 @@
 ext {
     isSnapshot = "true" == System.getProperty("build.snapshot", "true")
-    opensearch_version = System.getProperty("opensearch.version", "2.10.0")
+    opensearch_version = System.getProperty("opensearch.version", "2.11.1")
     plugin_version = opensearch_version
     if (isSnapshot) {
         opensearch_version += "-SNAPSHOT"