Update the Ruby quickstart sample (currently crashing on deploy)

[bourgeoisor]

The current Ruby samples is over 4 years old and now crashes on deploy.

kubectl logs helloworld-gke-67967444df-q48jd
/usr/local/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:54:in `require': cannot load such file -- rack/showexceptions (LoadError)
        from /usr/local/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:54:in `require'
        from /usr/local/bundle/gems/sinatra-1.0/lib/sinatra/showexceptions.rb:1:in `<top (required)>'
        from /usr/local/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:54:in `require'
        from /usr/local/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:54:in `require'
        from /usr/local/bundle/gems/sinatra-1.0/lib/sinatra/base.rb:6:in `<top (required)>'
        from /usr/local/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:54:in `require'
        from /usr/local/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:54:in `require'
        from /usr/local/bundle/gems/sinatra-1.0/lib/sinatra.rb:4:in `<top (required)>'
        from /usr/local/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:130:in `require'
        from /usr/local/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:130:in `rescue in require'
        from /usr/local/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_require.rb:34:in `require'
        from ./app.rb:1:in `<main>'

In the meantime, it's being removed from the quickstart doc (https://cloud.google.com/kubernetes-engine/docs/quickstarts/deploy-app-container-image) but we should look into updating this sample to be able to reinstate it.

https://github.com/GoogleCloudPlatform/kubernetes-engine-samples/tree/main/quickstart/ruby