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>