Use cgi/escape instead of cgi/util

[taketo1113]

Background

A warning is currently emitted in CI when running with ruby-head (Ruby 3.5-dev).
This warning occurs because CGI::Util will be removed in Ruby 3.5.

• https://bugs.ruby-lang.org/issues/21258
• Retire CGI library without cgi/escape ruby#13275

/home/runner/.rubies/ruby-head/lib/ruby/gems/3.5.0+2/gems/rss-0.3.2/lib/rss/rss.rb:4: warning: CGI::Util is removed from Ruby 3.5. Please use cgi/escape instead for CGI.escape and CGI.unescape features.
If you are using CGI.parse, please install and use the cgi gem instead.
/home/runner/work/rss/rss/tmp/test/test-dublincore.rb:2: warning: CGI library is removed from Ruby 3.5. Please use cgi/escape instead for CGI.escape and CGI.unescape features.
If you need to use the full features of CGI library, Please install cgi gem.

https://github.com/ruby/rss/actions/runs/16902042489/job/47883073913#step:6:8

Details

This Pull Request fixes the warning by replacing the use of cgi/util with cgi/escape.
The cgi/escape is available since Ruby 2.3.
In addition, for Ruby 3.4 and earlier, cgi/util is still required to use methods such as CGI.escape* and CGI.unescape* (especially CGI.escapeHTML in the rss gem).
(cgi/util was only required when using CGI.unescape method on Ruby 3.4 and earlier.)

Additional Information

Similar fixes have been made in other libraries, such as the rdoc gem.

• Load cgi/escape instead of cgi/util rdoc#1366
• Fixup to usage of cgi/escape for Windows CI rdoc#1378
• ruby/net-http@d8fd39c

[kou]

Thanks.