Drop dependency of dist target on ChangeLog

We no longer write manual ChangeLog entries since they are
auto-generated at release time.  Drop dependency of the `make dist`
target on the file and document the fact that the latest ChangeLog
entries can be read in the highest numbered ChangeLog.N file in
ChangeLog.old.

The ChangeLog.old/ChangeLog.20 file for 2.31 will thus be generated
just before tagging a release.

Reviewed-by: Paul Eggert <eggert@cs.ucla.edu>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>