Enable ci_child_of_child_pipeline by default

Also add a changelog for the feature

changelogs/unreleased/29651-243747-enable-child-of-child-pipeline.yml

+---
+title: Implement allowing child pipeline to have child pipeline
+merge_request: 42580
+author:
+type: added

config/feature_flags/development/ci_child_of_child_pipeline.yml

@@ -4,4 +4,4 @@ introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/41102
 rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/243747
 group: 'group::continuous integration'
 type: development
-default_enabled: false
+default_enabled: true

doc/ci/parent_child_pipelines.md

@@ -153,10 +153,10 @@ This is [resolved in GitLab 12.10](https://gitlab.com/gitlab-org/gitlab/-/issues
 ## Nested child pipelines
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/29651) in GitLab 13.4.
-> - It's [deployed behind a feature flag](../user/feature_flags.md), disabled by default.
-> - It's disabled on GitLab.com.
-> - It's not recommended for production use.
-> - To use it in GitLab self-managed instances, ask a GitLab administrator to [enable it](#enable-or-disable-nested-child-pipelines). **(CORE ONLY)**
+> - It's [deployed behind a feature flag](../user/feature_flags.md), enabled by default.
+> - It's enabled on GitLab.com.
+> - It's recommended for production use.
+> - For GitLab self-managed instances, GitLab administrators can opt to [disable it](#enable-or-disable-nested-child-pipelines). **(CORE ONLY)**
 
 Parent and child pipelines were introduced with a maximum depth of one level of child
 pipelines, which was later increased to two. A parent pipeline can trigger many child
@@ -165,12 +165,11 @@ possible to trigger another level of child pipelines.
 
 ### Enable or disable nested child pipelines **(CORE ONLY)**
 
-Nested child pipelines with a depth of two are under development and not ready for
-production use. This feature is deployed behind a feature flag that is **disabled by default**.
-Enabling this feature allows child pipelines to trigger one more level of child pipelines.
-The second level of child pipelines cannot trigger any further child pipelines.
+Nested child pipelines with a depth of two are under development but ready for
+production use. This feature is deployed behind a feature flag that is **enabled by default**.
+
 [GitLab administrators with access to the GitLab Rails console](../administration/feature_flags.md)
-can enable it.
+can opt to disable it.
 
 To enable it:
 

lib/gitlab/ci/features.rb

@@ -59,7 +59,7 @@ module Gitlab
       end
 
       def self.child_of_child_pipeline_enabled?(project)
-        ::Feature.enabled?(:ci_child_of_child_pipeline, project, default_enabled: false)
+        ::Feature.enabled?(:ci_child_of_child_pipeline, project, default_enabled: true)
       end
 
       def self.trace_overwrite?