Fix jekyll serve failing with Ruby 3.0

bundle exec jekyll serve --future

fails with

/vendor/bundle/ruby/3.0.0/gems/jekyll-4.2.0/lib/jekyll/commands/serve/servlet.rb:3:in `require': cannot load such file -- webrick (LoadError)

Compare with https://github.com/jekyll/jekyll/issues/8523

Gemfile

@@ -8,6 +8,7 @@ gem 'rouge', '~> 3.26.0'
 gem 'jekyll-redirect-from', '~> 0.16.0'
 gem 'jekyll-feed', '~> 0.15.1'
 gem "jekyll-sitemap", "~> 1.4"
+gem "webrick", "~> 1.7"
 
 group :test do
     gem 'html-proofer', '~> 3.19.2'