Enable variables_in_include_section_ci feature by default

Removed YAML definition for the feature flag
Removed all references and associated specs
Docs updated to reflect the rollout issue

changelogs/unreleased/variables-in-include-enable.yml

+---
+title: Support Project variables in `include` section of `gitlab-ci.yml`
+merge_request: 52108
+author:
+type: added

config/feature_flags/development/variables_in_include_section_ci.yml

----
-name: variables_in_include_section_ci
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/50188/
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/294294
-milestone: '13.8'
-type: development
-group: group::compliance
-default_enabled: false

doc/ci/yaml/README.md

@@ -377,16 +377,10 @@ NOTE:
 Use merging to customize and override included CI/CD configurations with local
 definitions. Local definitions in `.gitlab-ci.yml` override included definitions.
 
-#### Variables with `include`
+#### Variables with `include` **(CORE ONLY)**
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/284883) in GitLab 13.8.
-> - 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-includepredefined-project-variables). **(CORE ONLY)**
-
-WARNING:
-This feature might not be available to you. Check the **version history** note above for details.
+> - [Feature flag removed](https://gitlab.com/gitlab-org/gitlab/-/issues/294294) in GitLab 13.9.
 
 You can [use some predefined variables in `include` sections](../variables/where_variables_can_be_used.md#gitlab-ciyml-file)
 in your `.gitlab-ci.yml`:
@@ -400,25 +394,6 @@ include:
 For an example of how you can include these predefined variables, and their impact on CI jobs,
 see the following [CI variable demo](https://youtu.be/4XR8gw3Pkos).
 
-##### Enable or disable include:predefined-project-variables **(CORE ONLY)**
-
-Use of predefined project variables in `include` section of `.gitlab-ci.yml` is under development and not ready for production use. It is
-deployed behind a feature flag that is **disabled by default**.
-[GitLab administrators with access to the GitLab Rails console](../../administration/feature_flags.md)
-can enable it.
-
-To enable it:
-
-```ruby
-Feature.enable(:variables_in_include_section_ci)
-```
-
-To disable it:
-
-```ruby
-Feature.disable(:variables_in_include_section_ci)
-```
-
 #### `include:local`
 
 `include:local` includes a file from the same repository as `.gitlab-ci.yml`.

lib/gitlab/ci/config/external/mapper.rb

@@ -99,8 +99,6 @@ module Gitlab
           end
 
           def expand_variables(data)
-            return data unless ::Feature.enabled?(:variables_in_include_section_ci)
-
             if data.is_a?(String)
               expand(data)
             else

spec/lib/gitlab/ci/config/external/mapper_spec.rb

@@ -323,20 +323,6 @@ RSpec.describe Gitlab::Ci::Config::External::Mapper do
           expect { subject }.to raise_error(described_class::AmbigiousSpecificationError)
         end
       end
-
-      context 'when feature flag is turned off' do
-        let(:values) do
-          { include: full_local_file_path }
-        end
-
-        before do
-          stub_feature_flags(variables_in_include_section_ci: false)
-        end
-
-        it 'does not expand the variables' do
-          expect(subject[0].location).to eq('$CI_PROJECT_PATH' + local_file)
-        end
-      end
     end
   end
 end