Merge branch 'mgill-rollout-default-merge-ref' into 'master'

Add rollout issue for default_merge_ref_for_diffs

See merge request gitlab-org/gitlab!47299

config/feature_flags/development/default_merge_ref_for_diffs.yml

 ---
 name: default_merge_ref_for_diffs
 introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/34472
-rollout_issue_url: 
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/276917
 milestone: '13.4'
 type: development
 group: group::source code