Add `validates :key, uniqueness: { scope: :pipeline_schedule_id }` in `Ci::PipelineScheduleVariable` model

What does this MR do?

This MR adds validates :key, uniqueness: { scope: :pipeline_schedule_id } to Ci::PipelineScheduleVariable model.

Are there points in the code the reviewer needs to double check?

Is this easy to understand for other devs?

Why was this MR needed?

Originnaly issued at https://gitlab.com/gitlab-org/gitlab-ce/issues/34805. This is one of significant technical debt of PipelineSchedule. Also, this blocks https://gitlab.com/gitlab-org/gitlab-ce/issues/34518

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] API support added
• [-] Tests added for this feature/bug
• Review
  • [-] Has been reviewed by UX
  • [-] Has been reviewed by Frontend
  • Has been reviewed by Backend
  • [-] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/issues/34805