Add webrick as Ruby 3 doesn’t include it

This may be fixable by upgrading Jekyll as it is now declared as a dependency.

Author: mrmanc

lib/github-pages/dependencies.rb

@@ -9,6 +9,7 @@ class Dependencies
       # Jekyll
       "jekyll" => "3.9.5",
       "jekyll-sass-converter" => "1.5.2",
+      "webrick" => "1.8.1",
 
       # Converters
       "kramdown" => "2.4.0",