Rename get_next_page to get_success_url

From django 4.1, this function has been renamed
(https://docs.djangoproject.com/en/4.2/releases/4.1/)

authens/views.py

@@ -138,8 +138,8 @@ class LogoutView(auth_views.LogoutView):
         else:
             self.cas_connected = False
 
-    def get_next_page(self):
-        next_page = super().get_next_page()
+    def get_success_url(self):
+        next_page = super().get_success_url()
         if self.cas_connected:
             cas_client = get_cas_client(self.request)