Enable improved diff highlighting by default

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/285464

Set the feature flag `improved_merge_diff_highlighting` to enabled
state by default.

changelogs/unreleased/16950_improve_highlighting_for_diffs.yml → changelogs/unreleased/285464_improve_highlighting_for_diffs.yml

 ---
 title: Improve highlighting for merge diffs
-merge_request: 52499
+merge_request: 53980
 author:
 type: added

config/feature_flags/development/improved_merge_diff_highlighting.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/299884
 milestone: '13.9'
 type: development
 group: group::source code
-default_enabled: false
+default_enabled: true

lib/gitlab/diff/highlight_cache.rb

@@ -77,7 +77,7 @@ module Gitlab
       private
 
       def version
-        if Feature.enabled?(:improved_merge_diff_highlighting, diffable.project)
+        if Feature.enabled?(:improved_merge_diff_highlighting, diffable.project, default_enabled: :yaml)
           NEXT_VERSION
         else
           VERSION

lib/gitlab/diff/inline_diff.rb

@@ -31,7 +31,7 @@ module Gitlab
         # Skip inline diff if empty line was replaced with content
         return if old_line == ""
 
-        if Feature.enabled?(:improved_merge_diff_highlighting, project)
+        if Feature.enabled?(:improved_merge_diff_highlighting, project, default_enabled: :yaml)
           CharDiff.new(old_line, new_line).changed_ranges(offset: offset)
         else
           deprecated_diff