[Backport 5.6.x] Changing azureUseDeprecatedCompletionsAPIForOldModels to default to true (#64371)

Changing azureUseDeprecatedCompletionsAPIForOldModels to default to
trueso that customers who upgrade don't have to change siteadmin
config
<img width="323" alt="image"
src="https://github.com/user-attachments/assets/a690b54b-aa43-43c3-9f7d-202f161866bc">



## Test plan

Tested locally 

## Changelog


 <br> Backport 3b16059 from #64347

Co-authored-by: Ara <arafat.da.khan@gmail.com>

Author: sourcegraph-release-bot

schema/site.schema.json

@@ -2990,7 +2990,7 @@
         "azureUseDeprecatedCompletionsAPIForOldModels": {
           "description": "Enables the use of the older completions API for select Azure OpenAI models.",
           "type": "boolean",
-          "default": false
+          "default": true
         },
         "fastChatModelMaxTokens": {
           "description": "The maximum number of tokens to use as client when talking to fastChatModel. If not set, clients need to set their own limit.",