Validate NOT NULL constraint on approval_merge_request_rules_users#project_id

What does this MR do and why?

Validate NOT NULL constraint on approval_merge_request_rules_users#project_id

Epic: &11670

There are no rows where project_id is NULL: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/41965/commands/128835

This is a large table (https://postgres.ai/console/gitlab/gitlab-production-main/sessions/41965/commands/128836) and I will shard it in two steps:

1- (previous MR) add a not null constraint with validate: false + prepare an async constraint validation - !199931 (merged) && !200377 (merged)

2- (this MR) validate project_id NOT NULL

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