Fix Django + cron using the wrong connector

[ewjoachim]

Closes #1158

We should not use with_connector. replace_connector is the way to go. with_connector has been documented for ages so there's no hurry to remove it, but at least now, it's clear.

I'll need to think about whether this is testable but I'm really not sure.

This PR makes it so that we (almost) don't use with_connectors anymore.

Successful PR Checklist:

• [ ] Tests
  • [ ] (not applicable?)
• [x] Documentation
  • [ ] (not applicable?)

PR label(s):

• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20breaking%20%F0%9F%92%A5
• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20feature%20%E2%AD%90%EF%B8%8F
• [x] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20bugfix%20%F0%9F%95%B5%EF%B8%8F
• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20miscellaneous%20%F0%9F%91%BE
• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20dependencies%20%F0%9F%A4%96
• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20documentation%20%F0%9F%93%9A

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Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
procrastinate
app.py
procrastinate/contrib/django/management/commands
procrastinate.py
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