Update anonymous feedback feature flag

Update the anonymous_visual_review_feedback feature flag to have the
testing group as the group and the introduced by MR URL.

ee/config/feature_flags/development/anonymous_visual_review_feedback.yml

 ---
 name: anonymous_visual_review_feedback
-introduced_by_url: 
-rollout_issue_url: 
-group: 
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/18710
+rollout_issue_url:
+group: group::testing
 type: development
 default_enabled: false