Set OpenAI Model to gpt-4o-mini and Updated Spec Defaults

The new default model for OpenAI is gpt-4o, but gpt-4o-mini is much cheaper and should be the default. This change only updates the documentation, but the change from 4-o to 4-o-mini should also be  applied in https://github.com/dapr/components-contrib/blob/main/conversation/openai/metadata.yaml (currently incorrectly on 4-turbo)

Signed-off-by: Erin La <107987318+giterinhub@users.noreply.github.com>

Author: giterinhub

daprdocs/content/en/reference/components-reference/supported-conversation/openai.md

@@ -20,7 +20,7 @@ spec:
   - name: key
     value: mykey
   - name: model
-    value: gpt-4-turbo
+    value: gpt-4o-mini
   - name: cacheTTL
     value: 10m
 ```
@@ -34,9 +34,9 @@ The above example uses secrets as plain strings. It is recommended to use a secr
 | Field              | Required | Details | Example |
 |--------------------|:--------:|---------|---------|
 | `key`   | Y | API key for OpenAI. | `mykey` |
-| `model` | N | The OpenAI LLM to use. Defaults to `gpt-4-turbo`.  | `gpt-4-turbo` |
+| `model` | N | The OpenAI LLM to use. Defaults to `gpt-4o`.  | `gpt-4o` |
 | `cacheTTL` | N | A time-to-live value for a prompt cache to expire. Uses Golang duration format.  | `10m` |
 
 ## Related links
 
-- [Conversation API overview]({{< ref conversation-overview.md >}})
+- [Conversation API overview]({{< ref conversation-overview.md >}})