chore(deps): update pre-commit hook adamchainz/django-upgrade to v1.20.0

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
adamchainz/django-upgrade	repository	minor	1.19.0 -> 1.20.0

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Release Notes

adamchainz/django-upgrade (adamchainz/django-upgrade)

### [`v1.20.0`](https://togithub.com/adamchainz/django-upgrade/blob/HEAD/CHANGELOG.rst#1200-2024-07-19) [Compare Source](https://togithub.com/adamchainz/django-upgrade/compare/1.19.0...1.20.0) - Fix the `admin_register` fixer to avoid rewriting when there are duplicate `ModelAdmin` classes in the file. `Issue #​471 `\__.

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Configuration

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