Use collections.list_all instead of collections._get_all (#921)

integrations/weaviate/src/haystack_integrations/document_stores/weaviate/document_store.py

@@ -203,7 +203,7 @@ def client(self):
         self._client.connect()
 
         # Test connection, it will raise an exception if it fails.
-        self._client.collections._get_all(simple=True)
+        self._client.collections.list_all(simple=True)
         if not self._client.collections.exists(self._collection_settings["class"]):
             self._client.collections.create_from_dict(self._collection_settings)