Move entries older than last year to changelog-old

The current changelog is *more than twice as long* as the "archive".
So it could really use some trimming. I propose to always keep the
current and previous years' entries.

Second half of the change: Remove the old entries (2012 and older).
[To be merged once changelog-old has been updated](https://github.com/jenkinsci/backend-jenkins-ci.org-webcontents/pull/1).