docs: release-technician-checklist: update to leaf tree version pinning

docs: release-technician-checklist: update to leaf tree version pinning

Our releases look to flip-flop between keeping or discarding the date
and title of the referenced qemu-trad commit. I think with the hash
replaced by a tag, the commit's date and title would better also be
purged.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Jackson <iwj@xenproject.org>