[maven-release-plugin] prepare release STRUTS_2_3_15_2

git-svn-id: https://svn.apache.org/repos/asf/struts/struts2/branches/STRUTS_2_3_15_1_X@1524305 13f79535-47bb-0310-9956-ffa450edef68
[maven-release-plugin] prepare for next development iteration

git-svn-id: https://svn.apache.org/repos/asf/struts/struts2/branches/STRUTS_2_3_15_1_X@1524307 13f79535-47bb-0310-9956-ffa450edef68