Enable ci_new_artifact_file_reader by default

Also add a changelog for the bug fix

changelogs/unreleased/212373-249588-enable-ci_new_artifact_file_reader.yml

+---
+title: Fix triggering multiple children pipeline with the same artifact
+merge_request: 42595
+author:
+type: fixed

config/feature_flags/development/ci_new_artifact_file_reader.yml

@@ -4,4 +4,4 @@ introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/40268
 rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/249588
 group: group::pipeline authoring
 type: development
-default_enabled: false
+default_enabled: true

lib/gitlab/ci/features.rb

@@ -80,7 +80,7 @@ module Gitlab
       end
 
       def self.new_artifact_file_reader_enabled?(project)
-        ::Feature.enabled?(:ci_new_artifact_file_reader, project, default_enabled: false)
+        ::Feature.enabled?(:ci_new_artifact_file_reader, project, default_enabled: true)
       end
     end
   end