ChangeLog: Move all ChangeLog files to .pre-gitlog

From v1.16.1 onwards, Wget no longer maintains an active ChangeLog file.
Instead the ChangeLog will be automatically generated on each release
through gnulib's gitlog-to-changelog script. However, the old versions
of the ChangeLog files are retained for reference. These files are
renamed with a .pre-gitlog appended to their filenames.
Also removed ChangeLog.README file which is not required anymore