NEWS: rework the description of systemd-measure a bit again

NEWS: rework the description of systemd-measure a bit again

Try to separate the description so that changes are described first, and the
discussion follows separately. Remove some repeated verbose descriptions of the
subject: if one sentence describes that UKI contains an signature and describes
it in detail, the next sentence can just say "the signature" without
elaborating. Also, we don't do version-keying yet, so don't say "future"
kernels — older kernels will work too.