[ci/fix] Explicitly uninstall Django before installing required version

In the "Install test dependencies" step,
the CI installs Django 4.2, which results in an
"ImportError: cannot import name 'DEFAULT_STORAGE_ALIAS' from 'django.conf'".
during the QA checks step.

To avoid this, we need to explicitly uninstall
Django before installing the required version.

.github/workflows/ci.yml

@@ -78,7 +78,8 @@ jobs:
       - name: Install openwisp-monitoring
         run: |
           pip install -U -I -e .
-          pip install --force-reinstall ${{ matrix.django-version }}
+          pip uninstall -y django
+          pip install ${{ matrix.django-version }}
 
       - name: QA checks
         run: |