Replace epics issue_id foreign key with new key nullifies on delete

What does this MR do and why?

Related to #436381 (closed)

In !139788 (merged) we added the column issue_id to epics with a foreign key using on_delete: :cascade but we don't want to delete the epic in case of issue deletion so we need to add a new foreign key that nullifies the column instead.

This MR follows the steps recommended for updating foreign keys:

1. Add the new foreign key without validation 20231227103059_replace_fk_on_epics_issue_id.rb

   Migrate

   ❯ bin/rails db:migrate:up:main VERSION=20231227103059
   main: == [advisory_lock_connection] object_id: 182380, pg_backend_pid: 18428
   main: == 20231227103059 ReplaceFkOnEpicsIssueId: migrating ==========================
   main: -- transaction_open?(nil)
   main:    -> 0.0000s
   main: -- transaction_open?(nil)
   main:    -> 0.0000s
   main: -- execute("ALTER TABLE epics ADD CONSTRAINT fk_epics_issue_id_with_on_delete_nullify FOREIGN KEY (issue_id) REFERENCES issues (id) ON DELETE SET NULL NOT VALID;")
   main:    -> 0.0020s
   main: == 20231227103059 ReplaceFkOnEpicsIssueId: migrated (0.1396s) =================

   Rollback

   ❯ bin/rails db:migrate:down:main VERSION=20231227103059
   main: == [advisory_lock_connection] object_id: 182480, pg_backend_pid: 17847
   main: == 20231227103059 ReplaceFkOnEpicsIssueId: reverting ==========================
   main: -- transaction_open?(nil)
   main:    -> 0.0000s
   main: -- remove_foreign_key(:epics, {:column=>:issue_id, :on_delete=>:nullify, :name=>:fk_epics_issue_id_with_on_delete_nullify})
   main:    -> 0.0027s
   main: == 20231227103059 ReplaceFkOnEpicsIssueId: reverted (0.1152s) =================

2. Validate the new foreign key 20231227104408_validate_fk_epics_issue_id_with_on_delete_nullify.rb

   Migrate

   ❯ bin/rails db:migrate:up:main VERSION=20231227104408
   main: == [advisory_lock_connection] object_id: 182380, pg_backend_pid: 19692
   main: == 20231227104408 ValidateFkEpicsIssueIdWithOnDeleteNullify: migrating ========
   main: -- execute("SET statement_timeout TO 0")
   main:    -> 0.0003s
   main: -- execute("ALTER TABLE epics VALIDATE CONSTRAINT fk_epics_issue_id_with_on_delete_nullify;")
   main:    -> 0.0052s
   main: -- execute("RESET statement_timeout")
   main:    -> 0.0007s
   main: == 20231227104408 ValidateFkEpicsIssueIdWithOnDeleteNullify: migrated (0.1454s)
   
   main: == [advisory_lock_connection] object_id: 182380, pg_backend_pid: 19692

   Rollback

   ❯ bin/rails db:migrate:down:main VERSION=20231227104408
   main: == [advisory_lock_connection] object_id: 182440, pg_backend_pid: 19180
   main: == 20231227104408 ValidateFkEpicsIssueIdWithOnDeleteNullify: reverting ========
   main: == 20231227104408 ValidateFkEpicsIssueIdWithOnDeleteNullify: reverted (0.0040s)
   
   main: == [advisory_lock_connection] object_id: 182440, pg_backend_pid: 19180

3. Remove the old foreign key 20231227104711_remove_fk_epics_issue_id.rb

   Migrate

   ❯ bin/rails db:migrate:up:main VERSION=20231227104711
   main: == [advisory_lock_connection] object_id: 182380, pg_backend_pid: 21678
   main: == 20231227104711 RemoveFkEpicsIssueId: migrating =============================
   main: -- transaction_open?(nil)
   main:    -> 0.0000s
   main: -- remove_foreign_key(:epics, {:column=>:issue_id, :on_delete=>:cascade, :name=>:fk_893ee302e5})
   main:    -> 0.0034s
   main: == 20231227104711 RemoveFkEpicsIssueId: migrated (0.1490s) ====================
   
   main: == [advisory_lock_connection] object_id: 182380, pg_backend_pid: 21678

   Rollback

   ❯ bin/rails db:migrate:down:main VERSION=20231227104711
   main: == [advisory_lock_connection] object_id: 189220, pg_backend_pid: 22201
   main: == 20231227104711 RemoveFkEpicsIssueId: reverting =============================
   main: -- transaction_open?(nil)
   main:    -> 0.0000s
   main: -- transaction_open?(nil)
   main:    -> 0.0000s
   main: -- execute("ALTER TABLE epics ADD CONSTRAINT fk_893ee302e5 FOREIGN KEY (issue_id) REFERENCES issues (id) ON DELETE CASCADE NOT VALID;")
   main:    -> 0.0014s
   main: == 20231227104711 RemoveFkEpicsIssueId: reverted (0.1470s) ====================
   
   main: == [advisory_lock_connection] object_id: 189220, pg_backend_pid: 22201

Note that the column epics.issue_id is not yet in use, it will start being used with !140232 (merged).

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