maint: update NEWS.

* NEWS: Update with user visible changes since previous release.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
slingshot: sync with upstream.

* slingshot: Update to latest version for recent bootstrap fixes,
among others.
* bootstrap: Regenerate.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
Release version 31

* NEWS: Record release date.
maint: post-release administrivia.

* configure.ac (AC_INIT): Bump revision to 32.
* NEWS: Add header line for next release.
* .prev-version: Record previous version.
* ./local.mk (old_NEWS_hash): Auto-update.

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