Restructure the announcement goal to make it easier to add other issue management systems

The AnnouncementMojo is currently very hardwired to create announcements based on a changes.xml file. On top of that support was added to allow using JIRA instead. Later the option to merge release info from both changes.xml and JIRA was included.

The design of this isn't at all extendable. If you wanted to add another issue management system you would have to add several configuration options just to handle which issue management system(s) should be used. I plan to deprecate the parameters boolean generateJiraAnnouncement and boolean jiraMerge and to add a new parameter List issueManagementSystems to replace them.