GraphQL fetch stage by id

Ref: gitlab-com/gl-infra/production#7774 (closed)

What does this MR do and why?

This MR allows to fetch stage by id using ci_stage record instead of reading from ci_builds.stage which will get deprecated.

Why are we doing this?

Following up !97660 (merged) where we made sure ci_stage record for pages deployments was set up correctly we forgot to update ci_builds.stage leading to an inconsistency.

The frontend was unable to retrieve stages on job page as you can see on this example.

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