Merge branch 'ph/defaultTrueDisplayOutdatedLineDiffFlag' into 'master'

Enable display_outdated_line_diff by default

See merge request gitlab-org/gitlab!76887

config/feature_flags/development/display_outdated_line_diff.yml

 ---
 name: display_outdated_line_diff
 introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/72597
-rollout_issue_url:
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/345491
 milestone: '14.5'
 type: development
 group: group::code review
-default_enabled: false
+default_enabled: true