Draft: Resolve "Support merged results for as-if-jh pipeline"

From Draft to Ready

• Testing commit is removed (!118907 (22c69d24))
• We only have one merged results pipeline triggered for the corresponding branch in the JH validation project
• Move the start-as-if-jh job outside of the prepare stage, now that it's a waiting job. How about having a JiHu stage where we could put add-jh-files, prepare-as-if-jh-branch, sync-as-if-jh-branch?
• Update the documentation
• Extract the "trigger pipeline" logic to a (testable) Ruby class
• Think hard about whether we need the merge results pipelines in the light of the trade-offs we would need to make? (see context in Slack - internal)
• (Future iteration) Implement the triage-ops processor that’ll close the JiHu validation MRs when others are closed as well (and close manually the ones that were created beforehand)

Context

Related to #386585 (closed)

What does this MR do and why?

• Create an MR in the JH validation project in the start-as-if-jh job, and triggers a merged result pipeline for it.
• Remove the FORCE_GITLAB_CI flag when triggering the JH validation pipeline (legacy flag)

Open questions

• In prepare-as-if-jh-branch, I sometimes saw that we were pushing with the root user... Probably too much access for it right?

MR acceptance checklist

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