Add webrick to server gemfile

[ainar]

The current behavior of a server launch, with a clean install of Ruby 3, is a fail.

Top level ::CompositeIO is deprecated, require 'multipart/post' and use `Multipart::Post::CompositeReadIO` instead!
Top level ::Parts is deprecated, require 'multipart/post' and use `Multipart::Post::Parts` instead!
Could not find server "". 
Run `bin/rails server --help` for more options.

The reason is webrick is not included in Ruby since 3.0.0 : https://www.ruby-lang.org/en/news/2020/12/25/ruby-3-0-0-released/