Only run scheduled pipelines for ruby3 branch

What does this MR do and why?

Only run scheduled pipelines for ruby3 branch.

The new schedule is set at: https://gitlab.com/gitlab-org/gitlab/-/pipeline_schedules/311519/edit

I already enabled it because I think it's fine if we're running it without this merge request. It should cancel the push pipeline anyway. I deactivated it for now: !103656 (comment 1170721848)

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How to set up and validate locally

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MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #382043 (closed)