Preparing for release 0.23.6

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1434030 13f79535-47bb-0310-9956-ffa450edef68
HADOOP-8999. Move to incompatible section of changelog


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1434370 13f79535-47bb-0310-9956-ffa450edef68