GitLab CI: ensure Rubocop compliance
As explained in more details on #19307 (closed), without this:
- We keep regressing from the state we reached in #17646 (closed).
- I can't assume that any Rubocop violation I see in code I'm working on is a regression that I've introduced => alert fatigue and I start ignoring them altogether.
Based on !1277 (merged) (where I already fixed Rubocop offenses in 1 file), don't bother reviewing before that one is merged.
Closes #19307 (closed)
Edited by intrigeri