Enable unified diff lines flag by default

Enables the unified diff lines feature flag by default
ready for the removal of the feature flag all together.

app/controllers/projects/merge_requests/diffs_controller.rb

@@ -64,7 +64,7 @@ class Projects::MergeRequests::DiffsController < Projects::MergeRequests::Applic
       render: ->(partial, locals) { view_to_html_string(partial, locals) }
     }
 
-    options = additional_attributes.merge(diff_view: Feature.enabled?(:unified_diff_lines, @merge_request.project) ? "inline" : diff_view)
+    options = additional_attributes.merge(diff_view: Feature.enabled?(:unified_diff_lines, @merge_request.project, default_enabled: true) ? "inline" : diff_view)
 
     if @merge_request.project.context_commits_enabled?
       options[:context_commits] = @merge_request.recent_context_commits

app/controllers/projects/merge_requests_controller.rb

@@ -38,7 +38,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
     push_frontend_feature_flag(:approvals_commented_by, @project, default_enabled: true)
     push_frontend_feature_flag(:hide_jump_to_next_unresolved_in_threads, default_enabled: true)
     push_frontend_feature_flag(:merge_request_widget_graphql, @project)
-    push_frontend_feature_flag(:unified_diff_lines, @project)
+    push_frontend_feature_flag(:unified_diff_lines, @project, default_enabled: true)
     push_frontend_feature_flag(:highlight_current_diff_row, @project)
     push_frontend_feature_flag(:default_merge_ref_for_diffs, @project)
   end

config/feature_flags/development/unified_diff_lines.yml

@@ -4,4 +4,4 @@ introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/40131
 rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/241188
 group: group::source code
 type: development
-default_enabled: false
+default_enabled: true