Remove code related to ci_allow_failure_with_exit_codes feature flag

Removes everything related to ci_allow_failure_with_exit_codes feature
flag and updates documentation.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/292024

app/models/ci/build.rb

@@ -1124,7 +1124,6 @@ module Ci
     end
 
     def conditionally_allow_failure!(exit_code)
-      return unless ::Gitlab::Ci::Features.allow_failure_with_exit_codes_enabled?
       return unless exit_code
 
       if allowed_to_fail_with_code?(exit_code)

config/feature_flags/development/ci_allow_failure_with_exit_codes.yml

----
-name: ci_allow_failure_with_exit_codes
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/49145
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/292024
-milestone: '13.7'
-type: development
-group: group::pipeline authoring
-default_enabled: true

doc/ci/yaml/README.md

@@ -2318,13 +2318,7 @@ job3:
 #### `allow_failure:exit_codes`
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/273157) in GitLab 13.8.
-> - 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-allow_failureexit_codes). **(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/292024) in GitLab 13.9.
 
 Use `allow_failure:exit_codes` to dynamically control if a job should be allowed
 to fail. You can list which exit codes are not considered failures. The job fails
@@ -2348,25 +2342,6 @@ test_job_2:
       - 255
 ```
 
-##### Enable or disable `allow_failure:exit_codes` **(CORE ONLY)**
-
-`allow_failure:exit_codes` 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 disable it.
-
-To disable it:
-
-```ruby
-Feature.disable(:ci_allow_failure_with_exit_codes)
-```
-
-To enable it:
-
-```ruby
-Feature.enable(:ci_allow_failure_with_exit_codes)
-```
-
 ### `when`
 
 `when` is used to implement jobs that are run in case of failure or despite the

lib/gitlab/ci/config/entry/job.rb

@@ -186,8 +186,6 @@ module Gitlab
           private
 
           def allow_failure_criteria
-            return unless ::Gitlab::Ci::Features.allow_failure_with_exit_codes_enabled?
-
             if allow_failure_defined? && allow_failure_value.is_a?(Hash)
               allow_failure_value
             end

lib/gitlab/ci/features.rb

@@ -63,10 +63,6 @@ module Gitlab
         ::Feature.enabled?(:ci_pipeline_editor_page, project, default_enabled: :yaml)
       end
 
-      def self.allow_failure_with_exit_codes_enabled?
-        ::Feature.enabled?(:ci_allow_failure_with_exit_codes, default_enabled: :yaml)
-      end
-
       def self.rules_variables_enabled?(project)
         ::Feature.enabled?(:ci_rules_variables, project, default_enabled: true)
       end

lib/gitlab/ci/pipeline/seed/build.rb

@@ -188,7 +188,6 @@ module Gitlab
           # we need to prevent the exit codes from being persisted because they
           # would break the behavior defined by `rules:allow_failure`.
           def allow_failure_criteria_attributes
-            return {} unless ::Gitlab::Ci::Features.allow_failure_with_exit_codes_enabled?
             return {} if rules_attributes[:allow_failure].nil?
             return {} unless @seed_attributes.dig(:options, :allow_failure_criteria)
 

spec/lib/gitlab/ci/config/entry/job_spec.rb

@@ -763,16 +763,6 @@ RSpec.describe Gitlab::Ci::Config::Entry::Job do
         it 'returns allow_failure_criteria' do
           expect(entry.value[:allow_failure_criteria]).to match(exit_codes: [42])
         end
-
-        context 'with ci_allow_failure_with_exit_codes disabled' do
-          before do
-            stub_feature_flags(ci_allow_failure_with_exit_codes: false)
-          end
-
-          it 'does not return allow_failure_criteria' do
-            expect(entry.value.key?(:allow_failure_criteria)).to be_falsey
-          end
-        end
       end
     end
   end

spec/models/ci/build_spec.rb

@@ -4938,14 +4938,6 @@ RSpec.describe Ci::Build do
 
         it_behaves_like 'drops the build without changing allow_failure'
       end
-
-      context 'when ci_allow_failure_with_exit_codes is disabled' do
-        before do
-          stub_feature_flags(ci_allow_failure_with_exit_codes: false)
-        end
-
-        it_behaves_like 'drops the build without changing allow_failure'
-      end
     end
   end
 

spec/services/ci/create_pipeline_service/rules_spec.rb

@@ -145,20 +145,6 @@ RSpec.describe Ci::CreatePipelineService do
         expect(find_job('job-2').options.dig(:allow_failure_criteria)).to be_nil
         expect(find_job('job-3').options.dig(:allow_failure_criteria, :exit_codes)).to eq([42])
       end
-
-      context 'with ci_allow_failure_with_exit_codes disabled' do
-        before do
-          stub_feature_flags(ci_allow_failure_with_exit_codes: false)
-        end
-
-        it 'does not persist allow_failure_criteria' do
-          expect(pipeline).to be_persisted
-
-          expect(find_job('job-1').options.key?(:allow_failure_criteria)).to be_falsey
-          expect(find_job('job-2').options.key?(:allow_failure_criteria)).to be_falsey
-          expect(find_job('job-3').options.key?(:allow_failure_criteria)).to be_falsey
-        end
-      end
     end
 
     context 'if:' do