Move ChangeLog to ChangeLog.old/ChangeLog.19

We no longer maintain a manually-written ChangeLog file:

  <https://sourceware.org/ml/libc-alpha/2019-09/msg00333.html>
  <https://sourceware.org/ml/libc-alpha/2019-10/msg00131.html

>

Instead the release manager is expected to generate a ChangeLog-like
file using scripts/gitlog_to_changelog.py.  For further details,
see commit f2144b78 ("Script to
generate ChangeLog-like output from git log").

Reviewed-by: Carlos O'Donell <carlos@redhat.com>