This should be used wherever we need to show the latest release
date.
Don't use this in the release notes however, as it will be easy to
forget to update it when adding notes for a new release.
This brings the repmgr documentation build system in line with that
used by the main PostgreSQL project, and removed the restriction
that documentation must be built against PostgreSQL 9.6 or earlier.
Main formatting changes are:
- convert empty-element tags (mainly <xref/>)
- put <indexterm> sections in the correct location
- correct usage of various entities.
