Update dependency django-cleanup to v6

This MR contains the following updates:

Package	Type	Update	Change
django-cleanup	dependencies	major	^5.1.0 -> ^6.0.0

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Release Notes

un1t/django-cleanup

v6.0.0

Compare Source

Added

• Update to run tests for python 3.10. MR [#​88] from @​johnthagen.
• GitHub Actions. Resolves issue [#​89] for @​johnthagen.

Changed

• Fix default_app_config deprecation. MR [#​86] from @​nikolaik.

Removed

• Dropped support for django 3.0 and 3.1.
• Travis configuration.

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