Mark recommend_reviewers_assign as generally available

What does this merge request do and why?

Marks the Recommend Reviewers feature setting (recommend_reviewers_assign) as generally available. Its release state in feature_metadata.yml changes from BETA to GA, and it moves from FLAGGED_FEATURES to STABLE_FEATURES.

The self-hosted model selection page in the Admin area still showed a beta badge for Recommend Reviewers, and admins had to accept the GitLab Testing Terms before they could configure it. After this change, the badge goes away and the feature shows without the Testing Terms, like the other GA features.

References

Screenshots or screen recordings

How to set up and validate locally

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.