Set projects organization_id column to not null

What does this MR do and why?

Adds the backfill and constraint migrations to ensure projects' organization_id is not null.

Backfill Script

Postgres.ai Result (Zero Writes): https://console.postgres.ai/gitlab/gitlab-production-main/sessions/32698/commands/100836

Time: 3.476 ms
  - planning: 2.860 ms
  - execution: 0.616 ms
    - I/O read: 0.329 ms
    - I/O write: 0.000 ms

Shared buffers:
  - hits: 0 from the buffer pool
  - reads: 4 (~32.00 KiB) from the OS file cache, including disk I/O
  - dirtied: 0
  - writes: 0

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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How to set up and validate locally

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Related to #499665