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 psycopg2 driver explicitly.

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

related: #61229

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Generated-by: Claude Opus 4.6 following the guidelines

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[jscheffl]

Looks okay but I am only 90% sure as my Celery background is... limited.
Some second eye review?

[github-actions]

Backport failed to create: v3-1-test. View the failure log Run details

Note: As of Merging PRs targeted for Airflow 3.X
the committer who merges the PR is responsible for backporting the PRs that are bug fixes (generally speaking) to the maintenance branches.

In matter of doubt please ask in #release-management Slack channel.

Status	Branch	Result
❌	v3-1-test

You can attempt to backport this manually by running:

cherry_picker b4b5417 v3-1-test

This should apply the commit to the v3-1-test branch and leave the commit in conflict state marking
the files that need manual conflict resolution.

After you have resolved the conflicts, you can continue the backport process by running:

cherry_picker --continue

If you don't have cherry-picker installed, see the installation guide.