Enable FF ci_rules_variables by default

It's ready for production use

changelogs/unreleased/289803-enable-ci_rules_variables.yml

+---
+title: Implement variables for pipeline job rules
+merge_request: 50501
+author:
+type: added

config/feature_flags/development/ci_rules_variables.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/289803
 milestone: '13.7'
 type: development
 group: group::pipeline authoring
-default_enabled: false
+default_enabled: true

doc/ci/yaml/README.md

@@ -1437,10 +1437,11 @@ In this example, if the first rule matches, then the job has `when: manual` and
 #### `rules:variables`
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/209864) in GitLab 13.7.
-> - 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-rulesvariables). **(CORE ONLY)**
+> - It was [deployed behind a feature flag](../../user/feature_flags.md), disabled by default.
+> - [Became enabled by default](https://gitlab.com/gitlab-org/gitlab/-/issues/289803) on GitLab 13.8.
+> - 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-rulesvariables). **(CORE ONLY)**
 
 WARNING:
 This feature might not be available to you. Check the **version history** note above for details.
@@ -1467,10 +1468,10 @@ job:
 
 ##### Enable or disable rules:variables **(CORE ONLY)**
 
-rules:variables is under development and not ready for production use. It is
-deployed behind a feature flag that is **disabled by default**.
+rules:variables is under development but ready for production use.
+It 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

@@ -72,7 +72,7 @@ module Gitlab
       end
 
       def self.rules_variables_enabled?(project)
-        ::Feature.enabled?(:ci_rules_variables, project, default_enabled: false)
+        ::Feature.enabled?(:ci_rules_variables, project, default_enabled: true)
       end
     end
   end