Enable frozen string for lib/gitlab/ci

Enables frozen string for the following:

* lib/gitlab/ci/*.rb
* lib/gitlab/ci/build/**/*.rb
* lib/gitlab/ci/config/**/*.rb
* lib/gitlab/ci/pipeline/**/*.rb
* lib/gitlab/ci/reports/**/*.rb

Partially addresses #47424.

changelogs/unreleased/frozen-string-enable-lib-gitlab-ci.yml

+---
+title: Enable frozen string for lib/gitlab/ci
+merge_request:
+author: gfyoung
+type: performance

lib/gitlab/ci/ansi2html.rb

+# frozen_string_literal: true
+
 # ANSI color library
 #
 # Implementation per http://en.wikipedia.org/wiki/ANSI_escape_code
@@ -265,7 +267,7 @@ module Gitlab
         def reset_state
           @offset = 0
           @n_open_tags = 0
-          @out = ''
+          @out = +''
           reset
         end

lib/gitlab/ci/build/artifacts/adapters/gzip_stream.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Ci
     module Build

lib/gitlab/ci/build/artifacts/adapters/raw_stream.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Ci
     module Build

lib/gitlab/ci/build/artifacts/metadata.rb

+# frozen_string_literal: true
+
 require 'zlib'
 require 'json'

lib/gitlab/ci/build/artifacts/metadata/entry.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Ci
     module Build

lib/gitlab/ci/build/artifacts/path.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Ci
     module Build

lib/gitlab/ci/build/credentials/base.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Ci
     module Build

lib/gitlab/ci/build/credentials/factory.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Ci
     module Build

lib/gitlab/ci/build/credentials/registry.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Ci
     module Build

lib/gitlab/ci/build/image.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Ci
     module Build

lib/gitlab/ci/build/policy.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Ci
     module Build

lib/gitlab/ci/build/policy/kubernetes.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Ci
     module Build

lib/gitlab/ci/build/policy/refs.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Ci
     module Build

lib/gitlab/ci/build/policy/specification.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Ci
     module Build

lib/gitlab/ci/build/policy/variables.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Ci
     module Build

lib/gitlab/ci/build/step.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Ci
     module Build

lib/gitlab/ci/charts.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Ci
     module Charts

lib/gitlab/ci/config.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Ci
     #

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

+# frozen_string_literal: true
+
 module Gitlab
   module Ci
     class Config