Add and backfill project_id for p_ci_pipeline_variables

Ref: #463244 (closed)

What does this MR do and why?

This MR adds and backfills project_id for p_ci_pipeline_variables as this is a requirement for Cells1.0.

We have about ~2B records to update which should take ~6 weeks to complete on gitlab.com.

db/docs/batched_background_migrations/backfill_p_ci_pipeline_variables_project_id.yml

 ---
 migration_job_name: BackfillPCiPipelineVariablesProjectId
-description: Backfills sharding key `p_ci_pipeline_variables.project_id` from `p_ci_pipeline_variables`.
+description: Backfills sharding key `p_ci_pipeline_variables.project_id` from `p_ci_pipelines`.
 feature_category: continuous_integration
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/163445
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/164170
 milestone: '17.4'
 queued_migration_version: 20240828074159
 finalize_after: '2024-10-15'