maint: post-release administrivia

* NEWS: Add header line for next release.
* .prev-version: Record previous version.
* cfg.mk (old_NEWS_hash): Auto-update.
maint: move Peter O'Gorman and Ralf Wildenhues to past maintainers.

* AUTHORS: Move Peter O'Gorman and Ralf Wildenhues from current
to past maintainers list.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>