In Ruby 3.5, most of the cgi gem is removed which includes `CGI.parse`. We can replace this with `URI.decode_www_form` but it has some slight differences. Seems any blank value now always comes back as nil rather nil or an empty string depending so this can be a breaking change.

rouge-ruby/rouge#2131

* Replace CGI with URI for Ruby 3.5 support

In Ruby 3.5, most of the cgi gem is removed which includes `CGI.parse`. We can replace this with `URI.decode_www_form` but it has some slight differences. Seems any blank value now always comes back as nil rather nil or an empty string depending so this can be a breaking change.

rouge-ruby/rouge#2131

* Update CHANGELOG.md

This commit addresses this Rails CI Nightly failure below.
https://buildkite.com/rails/rails-nightly/builds/2679#0198a53d-2b95-4767-95aa-e54f1297493d/1193-1197

- Errors addressed by this commit

```
$ ruby -v
ruby 3.5.0dev (2025-08-12T14:43:46Z master c5c894c6e4) +PRISM [x86_64-linux]
$ cd guides
$ bundle install
$ bundle exec rake guides:lint
... snip ...
Generating active_record_validations.md as active_record_validations.html
Generating 5_1_release_notes.md as 5_1_release_notes.html
Generating asset_pipeline.md as asset_pipeline.html
/home/yahonda/.local/share/mise/installs/ruby/trunk/lib/ruby/gems/3.5.0+2/gems/rouge-4.5.1/lib/rouge/lexer.rb:55:in 'Rouge::Lexer.lookup_fancy': undefined method 'parse' for class CGI (NoMethodError)

        opts = CGI.parse(opts || '').map do |k, vals|
                  ^^^^^^
	from /home/yahonda/.local/share/mise/installs/ruby/trunk/lib/ruby/gems/3.5.0+2/gems/rouge-4.5.1/lib/rouge/lexer.rb:95:in 'Rouge::Lexer.find_fancy'
	from /home/yahonda/src/github.com/rails/rails/guides/rails_guides/markdown/renderer.rb:35:in 'RailsGuides::Markdown::Renderer#block_code'
	from /home/yahonda/src/github.com/rails/rails/guides/rails_guides/markdown.rb:90:in 'Redcarpet::Markdown#render'
	from /home/yahonda/src/github.com/rails/rails/guides/rails_guides/markdown.rb:90:in 'RailsGuides::Markdown#generate_body'
	from /home/yahonda/src/github.com/rails/rails/guides/rails_guides/markdown.rb:28:in 'RailsGuides::Markdown#render'
	from /home/yahonda/src/github.com/rails/rails/guides/rails_guides/generator.rb:214:in 'RailsGuides::Generator#generate_guide'
	from /home/yahonda/src/github.com/rails/rails/guides/rails_guides/generator.rb:108:in 'block in RailsGuides::Generator#generate_guides'
	from /home/yahonda/src/github.com/rails/rails/guides/rails_guides/generator.rb:106:in 'Array#each'
	from /home/yahonda/src/github.com/rails/rails/guides/rails_guides/generator.rb:106:in 'RailsGuides::Generator#generate_guides'
	from /home/yahonda/src/github.com/rails/rails/guides/rails_guides/generator.rb:52:in 'RailsGuides::Generator#generate'
	from rails_guides.rb:31:in '<main>'
rake aborted!
Command failed with status (1): [/home/yahonda/.local/share/mise/installs/ruby/trunk/bin/ruby -Eutf-8:utf-8 rails_guides.rb]
/home/yahonda/src/github.com/rails/rails/guides/Rakefile:33:in 'block (3 levels) in <top (required)>'
/home/yahonda/.local/share/mise/installs/ruby/trunk/bin/bundle:25:in '<main>'
Tasks: TOP => guides:lint => guides:lint:check_links
(See full trace by running task with --trace)
$
```

This workaround commit can be reverted when newer version of rouge gem
is relased including rouge-ruby/rouge#2131

- Ruby 3.5 removes cgi by default
https://bugs.ruby-lang.org/issues/21258
ruby/ruby#13275