Add constraints to spam_logs.user_id with NOT VALID

What does this MR do and why?

This change adds a NOT NULL constraint to the spam_logs.user_id column and removes records that refer to a non-existent user_id.

DELETE FROM "spam_logs"
WHERE (spam_logs.id) IN (
    SELECT spam_logs.id
    FROM "spam_logs"
      LEFT OUTER JOIN users ON spam_logs.user_id = users.id
    WHERE spam_logs.id >= 1
      AND spam_logs.id < 501
      AND users.id IS NULL
);

Time: 58.861 ms
  - planning: 2.562 ms
  - execution: 56.299 ms
    - I/O read: 54.627 ms
    - I/O write: 0.000 ms

Shared buffers:
  - hits: 1863 (~14.60 MiB) from the buffer pool
  - reads: 68 (~544.00 KiB) from the OS file cache, including disk I/O
  - dirtied: 0
  - writes: 0

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/45810/commands/140235

References

• #553470
• !212438 (comment 2887092004)

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