Add a workflow to sync commits to ruby/ruby

[k0kubun]

https://bugs.ruby-lang.org/issues/21628

[taketo1113]

It seems that the CI error on Ruby 2.4 is caused by the test-unit-ruby-core gem version 1.0.9.
https://github.com/ruby/ipaddr/actions/runs/18297568280/job/52098942424#step:4:16

Run bundle exec rake test
...
/home/runner/work/ipaddr/ipaddr/vendor/bundle/ruby/2.4.0/gems/test-unit-ruby-core-1.0.9/lib/core_assertions.rb:75:in `require_relative': /home/runner/work/ipaddr/ipaddr/vendor/bundle/ruby/2.4.0/gems/test-unit-ruby-core-1.0.9/lib/envutil.rb:108: syntax error, unexpected keyword_rescue, expecting keyword_end (SyntaxError)
      rescue Timeout::Error
            ^
/home/runner/work/ipaddr/ipaddr/vendor/bundle/ruby/2.4.0/gems/test-unit-ruby-core-1.0.9/lib/envutil.rb:481: syntax error, unexpected keyword_end, expecting end-of-input
...

I have confirmed that the tests pass on Ruby 2.4 after applying the following patch to the test-unit-ruby-core gem:
ruby/test-unit-ruby-core#18