[deps] Update django-nested-admin requirement from ~=4.1.4 to ~=4.1.6

Updates the requirements on [django-nested-admin](https://github.com/theatlantic/django-nested-admin) to permit the latest version.
- [Release notes](https://github.com/theatlantic/django-nested-admin/releases)
- [Changelog](https://github.com/theatlantic/django-nested-admin/blob/master/CHANGELOG.rst)
- [Commits](theatlantic/django-nested-admin@v4.1.4...v4.1.6)

---
updated-dependencies:
- dependency-name: django-nested-admin
  dependency-version: 4.1.6
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements.txt

@@ -1,4 +1,4 @@
 openwisp-controller @ https://github.com/openwisp/openwisp-controller/archive/refs/heads/1.3.tar.gz
 influxdb~=5.3.2
-django-nested-admin~=4.1.4
+django-nested-admin~=4.1.6
 python-dateutil>=2.7.0,<3.0.0