NEWS: announce plan to drop support for nscd

NEWS: announce plan to drop support for nscd

This is in preparation for https://github.com/systemd/systemd/pull/30360 to be
merged in a future release. As described there:

  nscd is known to be racy [1] and it was already deprecated and later dropped
  in Fedora a while back [1,2]. We don't need to support obsolete stuff in
  systemd, and the cache in systemd-resolved provides a better solution anyway.

  [1] https://fedoraproject.org/wiki/Changes/DeprecateNSCD
  [2] https://fedoraproject.org/wiki/Changes/RemoveNSCD

Note that our "support" is only the signal to flush the cache that we send at
various points. Nscd itself may still exist, dropping it is a decision to be
made in glibc.