[deps] Update django-taggit requirement from ~=4.0.0 to ~=6.0.0

Updates the requirements on [django-taggit](https://github.com/jazzband/django-taggit) to permit the latest version.
- [Changelog](https://github.com/jazzband/django-taggit/blob/master/CHANGELOG.rst)
- [Commits](jazzband/django-taggit@4.0.0...6.0.0)

---
updated-dependencies:
- dependency-name: django-taggit
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

requirements.txt

@@ -1,6 +1,6 @@
 django-sortedm2m~=3.1.1
 django-reversion~=5.0.12
-django-taggit~=4.0.0
+django-taggit~=6.0.0
 netjsonconfig @ https://github.com/openwisp/netjsonconfig/tarball/master
 django-x509 @ https://github.com/openwisp/django-x509/tarball/master
 django-loci @ https://github.com/openwisp/django-loci/tarball/master