update DEFAULT_OPENAI_GENERATE_MODEL because of deprication

`text-davinci-003` was [deprecated](https://platform.openai.com/docs/deprecations).

We noticed the issue when we were using a default model for contextual rephrasing in Studio.

This PR just changes a default to a new `gpt-3.5-turbo-instruct` model

rasa/utils/llm.py

@@ -10,7 +10,7 @@
 
 AI = "AI"
 
-DEFAULT_OPENAI_GENERATE_MODEL_NAME = "text-davinci-003"
+DEFAULT_OPENAI_GENERATE_MODEL_NAME = "gpt-3.5-turbo-instruct"
 
 DEFAULT_OPENAI_CHAT_MODEL_NAME = "gpt-3.5-turbo"