slingshot: sync with upstream, which requires lyaml 4+.

* slingshot: Pick up recent improvements, particularly bootstrap
bugfixes and revised backward-compatible lyaml 4 APIs.
* bootstrap: Upgrade from slingshot.
* rockspec.conf (dependencies): Require lyaml 4 or better.
* lib/specl.in: Adjust for lyaml 4 APIs.
* NEWS: Amend lyaml 4 compatibility note.

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