Gemfile: add 'webrick' dependency for Ruby 3.0.0 and above

Fixes carpentries/styles#552

Author: josephmje

Gemfile

@@ -8,3 +8,7 @@ git_source(:github) { |repo_name| "https://github.com/#{repo_name}" }
 ruby '>=2.7.1'
 
 gem 'github-pages', group: :jekyll_plugins
+
+if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('3.0.0')
+    gem 'webrick', '>= 1.6.1'
+end