cleanup before doing the migration

[mgerhardy]

on older (as in living longer) gitlab instances the pipelines will waste a lot of disk space for artifacts, logs and so on (even if you set the expire_in option for the artifacts in your .gitlab-ci.yml)

I've written a small script to clean those pipelines up - see https://gist.github.com/mgerhardy/a2b3f62c6bc3517536a4b998ea0e7ac7