Set `ai_merge_request_reader_for_chat` as default enabled

What does this MR do and why?

ai_merge_request_reader_for_chat has been rolled out on production for over a day now, so we can set it as default enabled so that it can be ready for 17.5.

MR acceptance checklist

✅

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

Related to #463499 (closed)