Update rollout issue link for display_merge_conflicts_in_diff flag

What does this MR do?

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

Does this MR meet the acceptance criteria?

Conformity

• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Relates to #276918 (closed)