GitLab CI: add a job that will fail if we need to merge from upstream

This is a first step towards tails#10582: instead of manually checking every quarter whether we need to do anything, this new CI job will check it for us.

I've configured a scheduled pipeline to run this every week. My understanding is that I'll be notified on failure, which is somewhat worse than an issue which every FT member can see like what we did so far (e.g. tails#18927 (closed)). I think that's good enough as a first iteration, and then at some point we can follow-up on tails#10582, for example:

• The cheapest way to improve this may be to make the owner of the scheduled pipeline be a role user (for example @ft-bot) whose email address is our team's mailing list.
• The best way to improve this may be to complete tails#10582: if there are upstream changes, create a MR that proposes merging upstream-repo/develop into our own devel branch; most of the time there will be merge conflicts to address, so the main benefit is to ensure this lands on our radar (FT is supposed to monitor open MRs).