Preparing for release 0.23.6

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1434031 13f79535-47bb-0310-9956-ffa450edef68
HADOOP-8999. Move to incompatible section of changelog


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1434373 13f79535-47bb-0310-9956-ffa450edef68