♻️ Clean up pool streams data

[esizer]

♻️ Debt/Refactor

After migrating pool streams to the new work stream model, we will need to clean up after ourselves and remove old data.

[!CAUTION] Do not merge this before #7574 has been fully deployed in prod and migration scripts have been run.

🙋‍♀️ Proposed Solution

Create migration that removes stream column from pools and job_poster_templates table and sets work_stream_id to non-nullable in job_poster_templates table.

Delete PoolStream enum code. If anything is left that references it, like keys in the WorkStream seeder, replace with hard coded strings.

✅ Acceptance Criteria

• [ ] Old pool streams data removed from database
• [ ] PoolStream enum code gone.
• [ ] The new work stream FK is non-nullable on the job_poster_templates table

🛑 Blockers

### Blocked By
- [ ] https://github.com/GCTC-NTGC/gc-digital-talent/issues/7574

[github-actions[bot]]

Status: Error :warning:

Issues blocking this PR:

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