chore(deps): update dependency django-upgrade to v1.21.0

This MR contains the following updates:

Package	Type	Update	Change
django-upgrade (changelog)	dev-dependencies	minor	1.19.0 -> 1.21.0

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Release Notes

adamchainz/django-upgrade (django-upgrade)

v1.21.0

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• Add Django 5.0+ fixer to rewrite format_html() calls without args or kwargs probably using str.format() incorrectly.

  Issue #&#8203;477 <https://github.com/adamchainz/django-upgrade/issues/477>__.

v1.20.0

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• Fix the admin_register fixer to avoid rewriting when there are duplicate ModelAdmin classes in the file.

  Issue #&#8203;471 <https://github.com/adamchainz/django-upgrade/issues/471>__.

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