Migrate the automatic merge pipeline from tails/gitlab-config to tails/tails

Goal: sysadmins are not on the critical path anymore for automation needed by Tails team in their tails/tails project.

In https://gitlab.tails.boum.org/tails/tails/-/issues/18003#note_250008, @zen offered to migrate the scheduled pipeline from https://gitlab.tails.boum.org/tails/gitlab-config/-/pipeline_schedules to the tails/tails project. Thanks!

Broader context: tails/tails#18003

• Add code to the tails repo: https://gitlab.tails.boum.org/tails/tails/-/merge_requests/1876
• Setup the scheduled pipeline in the tails repo using a token that doesn't expire
• Review/update the scheduled pipeline documentation (ref)
• Remove the scheduled pipeline and code from gitlab-config