Enable redis diff caching by default

changelogs/unreleased/kerrizor-enable-redis-diff-caching-by-default.yml

+---
+title: Enable redis HSET diff caching by default
+merge_request: 23105
+author:
+type: performance

lib/gitlab/diff/file_collection/merge_request_diff_base.rb

@@ -47,7 +47,7 @@ module Gitlab
         private
 
         def cache
-          @cache ||= if Feature.enabled?(:hset_redis_diff_caching, project)
+          @cache ||= if Feature.enabled?(:hset_redis_diff_caching, project, default_enabled: true)
                        Gitlab::Diff::HighlightCache.new(self)
                      else
                        Gitlab::Diff::DeprecatedHighlightCache.new(self)