Bump django-filter from 2.2.0 to 2.4.0

Bumps [django-filter](https://github.com/carltongibson/django-filter) from 2.2.0 to 2.4.0.
- [Release notes](https://github.com/carltongibson/django-filter/releases)
- [Changelog](https://github.com/carltongibson/django-filter/blob/master/CHANGES.rst)
- [Commits](carltongibson/django-filter@2.2.0...2.4.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements.txt

@@ -1,5 +1,5 @@
 modoboa>=1.15.0
-django-filter==2.2.0
+django-filter==2.4.0
 drf-nested-routers
 django-webpack-loader
 caldav==0.6.2