Finalize the DeleteNullProjectIdPushRules BBM for self-managed

Everyone can contribute. Help move this issue forward while earning points, leveling up and collecting rewards.

• Label this issue

Summary

Finalize the DeleteNullProjectIdPushRules batched background migration (#599768 (closed)) for self-managed instances. This removes all rows where project_id IS NULL from push_rules, which is the precondition for validating the NOT NULL constraint on self-managed (see follow-up issue for Task 3).

The BBM was already finalized for GitLab.com in #605241 (closed). This issue does the equivalent for self-managed, which must happen after the 19.5 required stop.

Parent tracking issue: #499156 (deferred Task 2).

Background

• BBM: #599768 (closed) (DeleteNullProjectIdPushRules)
• Finalize (.com): #605241 (closed) (already done)
• Must run after required stop 19.5 (self-managed upgrade path).
• Blocks self-managed constraint validation (Task 3).

Related issues

• Parent: #499156 (deferred Task 2)
• .com finalize: #605241 (closed)
• BBM: #599768 (closed)
• Blocks: self-managed validation (Task 3)