Swap FK ci_pipeline_schedules.project_id to projects for LFK

What does this MR do and why?

Per &7249 (closed)

As part of our CI "decomposition" efforts we need to remove all foreign keys that are cross-database (ie. between the planned main and ci databases). We are going to replace them all with "loose foreign keys".

Related: grouppipeline execution devopsverify

Validations

• Best team to review (check off when reviewed): grouppipeline execution devopsverify

• No way for user to access once parent is deleted. Please explain:

  For user interactions we load the schedules starting from the project:

  • https://gitlab.com/gitlab-org/gitlab/-/blob/b2c8345b4ae1f74bce6fe4be648cfbac2b1bd0b6/app/controllers/projects/pipeline_schedules_controller.rb#L93-95
  • https://gitlab.com/gitlab-org/gitlab/-/blob/b2c8345b4ae1f74bce6fe4be648cfbac2b1bd0b6/app/finders/ci/pipeline_schedules_finder.rb#L9
  • https://gitlab.com/gitlab-org/gitlab/-/blob/b2c8345b4ae1f74bce6fe4be648cfbac2b1bd0b6/lib/api/ci/pipeline_schedules.rb#L193-203

• Possible to access once parent deleted but low user impact. Please explain:

• Possible Sidekiq workers that may load directly and possibly lead to exceptions. Please explain:

  There is a Sidekiq worker that loads the schedules directly and creates pipelines: https://gitlab.com/gitlab-org/gitlab/-/blob/b2c8345b4ae1f74bce6fe4be648cfbac2b1bd0b6/app/workers/pipeline_schedule_worker.rb#L13-21

• Possible user impact to be evaluated or mitigated. Please explain:

• Is this FK safe to be removed to avoid LOCKing problems? (Explanation: &7249 (comment 819662046)). Please explain:

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.