Properly cast %(statuses)s::procrastinate_job_status[] for psycopg2

[ewjoachim]

Closes #1075

We didn't test delete_old_jobs with psycopg2. The rewriting of the query for psycopg3 made it incompatible. I'm not 100% surprised that it went so long without being detected, but at least now it's fixed.

Successful PR Checklist:

• [x] Tests
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• [ ] Documentation
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• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20breaking%20%F0%9F%92%A5
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