Add a `NOT VALID` foreign key on `Project#creator_id`

What does this MR do and why?

This MR originates from the discussion in #378476 (comment 1153919092)

This adds a ON DELETE SET NULL, but NOT VALID foreign key relationship in the Project#creator_id column, pointing to the User#id column.

On the model, this is the relationship between the two tables.

Why NOT VALID? - because there are existing records in the projects table that point to already deleted users, and that needs to be cleaned up (#378477 (closed)), before we can validate this constraint.

Follow up:

• To clean existing, incorrect records: #378477 (closed) (in %15.6)
• To validate the relationship: #379677 (closed) (in %15.7)

UP Migration

$ rails db:migrate

main: == 20221031102916 AddUsersForeignKeyToProjects: migrating =====================
main: -- transaction_open?()
main:    -> 0.0000s
main: -- foreign_keys(:projects)
main:    -> 0.0119s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- execute("ALTER TABLE projects\nADD CONSTRAINT fk_03ec10b0d3\nFOREIGN KEY (creator_id)\nREFERENCES users (id)\nON DELETE SET NULL\nNOT VALID;\n")
main:    -> 0.0031s
main: == 20221031102916 AddUsersForeignKeyToProjects: migrated (0.0273s) ============

ci: == 20221031102916 AddUsersForeignKeyToProjects: migrating =====================
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- foreign_keys(:projects)
ci:    -> 0.0057s
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- execute("ALTER TABLE projects\nADD CONSTRAINT fk_03ec10b0d3\nFOREIGN KEY (creator_id)\nREFERENCES users (id)\nON DELETE SET NULL\nNOT VALID;\n")
ci:    -> 0.0064s
ci: == 20221031102916 AddUsersForeignKeyToProjects: migrated (0.0136s) ============

DOWN Migration

$ rake db:migrate:down:main VERSION=20221031102916

main: == 20221031102916 AddUsersForeignKeyToProjects: reverting =====================
main: -- transaction_open?()
main:    -> 0.0000s
main: -- foreign_keys(:projects)
main:    -> 0.0080s
main: -- remove_foreign_key(:projects, {:column=>:creator_id})
main:    -> 0.0068s
main: == 20221031102916 AddUsersForeignKeyToProjects: reverted (0.0227s) ============

$ rake db:migrate:down:ci VERSION=20221031102916

ci: == 20221031102916 AddUsersForeignKeyToProjects: reverting =====================
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- foreign_keys(:projects)
ci:    -> 0.0091s
ci: -- remove_foreign_key(:projects, {:column=>:creator_id})
ci:    -> 0.0089s
ci: == 20221031102916 AddUsersForeignKeyToProjects: reverted (0.0251s) ============

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