Automatically create CE --> EE merge requests

As soon as release-tools!86 (closed) gets merged, I'd like to have a scheduled pipeline that

• creates merge requests from CE to EE automatically
• leaves the link in the #ce-to-ee Slack channel.
• creates no new merge request if one with CE upstream label is still pending

I think this would help getting the merges smaller, so they can be merged faster and the problematic merges have smaller diff.