Backport(v1.16): Stop using CGI.parse due to support Ruby 3.5 (#4962)

[Watson1978]

Backport #4962

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:
At Ruby 3.5, CGI.parse will be removed.
https://bugs.ruby-lang.org/issues/21258

Therefore, the test fails where CGI.parse is used.

[master][~/src/fluentd]$ ruby -v -I"lib:test" test/plugin_helper/http_server/test_request.rb -v
ruby 3.5.0dev (2025-05-17T00:42:12Z master aa0f689bf4) +PRISM [x86_64-linux]
/home/watson/.rbenv/versions/3.5-dev/lib/ruby/gems/3.5.0+1/gems/async-pool-0.10.3/lib/async/pool/controller.rb:331: warning: assigned but unused variable - error
/home/watson/.rbenv/versions/3.5-dev/lib/ruby/gems/3.5.0+1/gems/traces-0.15.2/lib/traces/config.rb:44: warning: assigned but unused variable - error
/home/watson/src/fluentd/lib/fluent/plugin_helper/http_server/request.rb:17: 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.
/home/watson/src/fluentd/lib/fluent/plugin_helper.rb:46: warning: method redefined; discarding old inherited
/home/watson/src/fluentd/lib/fluent/plugin_helper.rb:46: warning: previous definition of inherited was here
Loaded suite test/plugin_helper/http_server/test_request
Started
HttpHelperRequestTest:
  test_request:                                                                                                                 E
================================================================================================================================================
Error: test_request(HttpHelperRequestTest): NoMethodError: undefined method 'parse' for class CGI
/home/watson/src/fluentd/lib/fluent/plugin_helper/http_server/request.rb:38:in 'Fluent::PluginHelper::HttpServer::Request#query'
test/plugin_helper/http_server/test_request.rb:14:in 'HttpHelperRequestTest#test_request'
     11:
     12:     assert_equal('/path', request.path)
     13:     assert_equal('foo=42', request.query_string)
  => 14:     assert_equal({'foo'=>['42']}, request.query)
     15:     assert_equal('text/html', request.headers['content-type'])
     16:     assert_equal(['gzip'], request.headers['content-encoding'])
     17:   end
================================================================================================================================================
: (0.005199)

Finished in 0.005372395 seconds.
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1 tests, 2 assertions, 0 failures, 1 errors, 0 pendings, 0 omissions, 0 notifications
0% passed
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186.14 tests/s, 372.27 assertions/s

This patch will replace CGI.parse with URI.decode_www_form

Docs Changes:
Not needed.

Release Note:

• in_monitor_agent: stop using CGI.parse due to support Ruby 3.5
• HTTP server plugin helper: stop using CGI.parse due to support Ruby 3.5