Add scheduled_at_before scope to CommitStatus

What does this MR do and why?

This MR ties up a query we use to get scheduled timed-out builds in an AR scope so it can be neatly reused.

We prefer this structure as a rule, and in doing this we can get rid of a rubocop exception in the implementing service class.

This merge request is dependent on !71229 (merged), but we can't officially mark it as such yet.

This is a follow-up change to work for #334399 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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