Update rollout issue link for display_merge_conflicts_in_diff flag

The old one is already closed and we're doing the roll out in the
new issue.

config/feature_flags/development/display_merge_conflicts_in_diff.yml

 ---
 name: display_merge_conflicts_in_diff
 introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/45008
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/277097
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/276918
 milestone: '13.5'
 type: development
 group: group::code review