Celery: explicitly specify psycopg2 in driverless postgres URLs

[Dev-iL]

This PR resolves a breaking change introduced in SQLA2.1 where the default postgres driver was changed from psycopg2 (v2) to psycopg (v3).

The fix is to specify the psychopg2 driver explicitly.

To use the v3 driver, users should specify it explicitly: postgresql+psycopg://...

related: #61229

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