Enable duo_code_review_custom_instructions by default

What does this MR do and why?

• duo_code_review_custom_instructions feature flag is enabled for everyone on gitlab.com
• This MR enables duo_code_review_custom_instructions feature flag by default. This means two things:
  • DCR Custom Instructions feature gets released to everyone including self-managed customers
  • DCR uses Claude 4

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #545136 (closed)