[deps] Update django-reversion requirement from ~=5.1.0 to ~=6.0.0

Updates the requirements on [django-reversion](https://github.com/etianen/django-reversion) to permit the latest version.
- [Release notes](https://github.com/etianen/django-reversion/releases)
- [Changelog](https://github.com/etianen/django-reversion/blob/master/CHANGELOG.rst)
- [Commits](etianen/django-reversion@v5.1.0...v6.0.0)

---
updated-dependencies:
- dependency-name: django-reversion
  dependency-version: 6.0.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Federico Capoano <f.capoano@openwisp.io>

Author: dependabot[bot]

requirements.txt

@@ -1,4 +1,4 @@
 openwisp-users @ https://github.com/openwisp/openwisp-users/tarball/1.2
 openwisp-utils[rest] @ https://github.com/openwisp/openwisp-utils/tarball/1.2
-django-reversion~=5.1.0
+django-reversion~=6.0.0
 openpyxl~=3.1.5