Swap FK project_repositories.project_id to projects.id for LFK

Summary

As part of our "sharding key" efforts, we need to mark the project_repositories as a Cell-local table shifting it to the gitlab_main_cell schema. But it has a foreign key constraint to a table in other schemas, e.g. gitlab_main_cell, or gitlab_ci_cell. The table which foreign key references is: projects., We are going to replace the foreign keys with "loose foreign keys".