Update django-sortedm2m requirement from ~=3.1 to ~=4.0

Updates the requirements on [django-sortedm2m](https://github.com/jazzband/django-sortedm2m) to permit the latest version.
- [Release notes](https://github.com/jazzband/django-sortedm2m/releases)
- [Changelog](https://github.com/jazzband/django-sortedm2m/blob/master/CHANGES.rst)
- [Commits](jazzband/django-sortedm2m@3.1.0...4.0.0)

---
updated-dependencies:
- dependency-name: django-sortedm2m
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements.txt

@@ -43,4 +43,4 @@ djangorestframework-simplejwt[crypto]==5.3.1
 
 # Not used anymore, but needed because some modules are imported in DB migration
 # files
-django-sortedm2m~=3.1
+django-sortedm2m~=4.0