doc: only distribute 5 years of ChangeLogs

doc: only distribute 5 years of ChangeLogs

Remove old log files that have corresponding entries
in the source code repository.
This saves about 2.5MB uncompressed, 0.5M compressed.

* Makefile.am (gen-ChangeLog): Adjust to taking all
logs since a particular version (8.15 in this case).
Also mention in the truncated log where to get older entries.
(changelog_etc): Remove the no longer distributed files.
* build-aux/git-log-fix: Remove now unused entries.
* ChangeLog-200[5-8]: Delete.
* doc/ChangeLog-2007: Likewise.
* po/ChangeLog-2007: Likewise.
* old/*: Likewise.