Add stage name in job.json response

What does this MR do?

Backend

In the job view, frontend need to know which stage the current job belongs to, so in the sidebar where we show the jobs related to that stage they know which stage to get. Instead of having frontend looping through all the stages and match the current job.

As discussed in https://gitlab.com/gitlab-org/gitlab-ce/issues/52618#note_108344867 & https://gitlab.com/gitlab-org/gitlab-ce/issues/52618#note_108353863

What are the relevant issue numbers?

Fix for gitlab-org/gitlab-ce#52618

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

Closes #52618 (closed)