Fix all YAML warnings in gitlab project
What does this MR do and why?
The YAML warnings have slowly built up over the last few months, and we should clean them up. The main reason is that when the linter throws an error and fails the pipeline, it can be hard to see why the job is failing. For example: https://gitlab.com/gitlab-org/gitlab/-/jobs/3272724076
See also the comments about it being hard to see the problem:
This MR fixes all the warnings shown here: https://gitlab.com/gitlab-org/gitlab/-/jobs/3273266581
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