WIP: Rename the domain model from `Ci::Variable` to `Ci::ProjectVariable`

What does this MR do?

This MR renames the domain model from Ci::Variable to Ci::ProjectVariable

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

This MR should be merged after https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12448 merged.

Why was this MR needed?

Decouling from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11078.

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
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #33980 (moved)