[maven-release-plugin] prepare release org.apache.sling.api-2.8.0

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1621068 13f79535-47bb-0310-9956-ffa450edef68
[maven-release-plugin] prepare for next development iteration

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1621070 13f79535-47bb-0310-9956-ffa450edef68