Set back similarity fn to null for backward compatibility

config_sentence_transformers.json

@@ -6,7 +6,7 @@
   },
   "prompts": {},
   "default_prompt_name": null,
-  "similarity_fn_name": "MaxSim",
+  "similarity_fn_name": null,
   "query_prefix": "[unused0]",
   "document_prefix": "[unused1]",
   "query_length": 32,