Default enable generate_commit_message_claude_3_7

What does this MR do and why?

Default enable generate_commit_message_claude_3_7 feature flag. The feature flag was created as a experiment so we need to change it to beta first before we could default enable it. It has been enabled on gitlab.com since 11th March.

References

#522917 (closed)

https://gitlab.com/gitlab-org/gitlab/-/issues/521392

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 #522917 (closed)