enable auto-abandon of stale changes

gerrit should be configured to auto-abandon changes that have been stale for three months (we can make it six months at first, but tighten it in a few weeks).

the message should be "Automatic cleanup after prolonged inactivity.
See https://wiki.qt.io/Gerrit_Auto_Cleanup for more information." (as my old script does). the wiki needs minor adjustments, as the "actor" will be different.