community: replace deprecated davinci models (#14860)

This is technically a breaking change because it'll switch out default
models from `text-davinci-003` to `gpt-3.5-turbo-instruct`, but OpenAI
is shutting off those endpoints on 1/4 anyways.

Feels less disruptive to switch out the default instead.

cookbook/learned_prompt_optimization.ipynb

@@ -51,7 +51,7 @@
     "\n",
     "from langchain.llms import OpenAI\n",
     "\n",
-    "llm = OpenAI(model=\"text-davinci-003\")"
+    "llm = OpenAI(model=\"gpt-3.5-turbo-instruct\")"
    ]
   },
   {

cookbook/tree_of_thought.ipynb

@@ -26,7 +26,7 @@
    "source": [
     "from langchain.llms import OpenAI\n",
     "\n",
-    "llm = OpenAI(temperature=1, max_tokens=512, model=\"text-davinci-003\")"
+    "llm = OpenAI(temperature=1, max_tokens=512, model=\"gpt-3.5-turbo-instruct\")"
    ]
   },
   {