Using only major version in docs metadata

otherwise Google is confused how to find docs for a given v. of Cayenne

(cherry picked from commit 212afab6627714e8db6061af980db663489994ce)

git-svn-id: https://svn.apache.org/repos/asf/cayenne/main/branches/STABLE-3.1@1555531 13f79535-47bb-0310-9956-ffa450edef68