Fix compatibility with Ruby 3.0

Author: iamed2

Gemfile

@@ -1,3 +1,8 @@
 source 'https://rubygems.org'
 
 gem 'github-pages', group: :jekyll_plugins
+
+# This must added since upgrading to Ruby 3.0.0 (https://github.com/jekyll/jekyll/issues/8523). This
+#   will most likely be fixed in a future version of Jekyll, but that'll be in version 4.3.0 (most
+#   likely), and this project is still using Jekyll 3.9.0.
+gem "webrick"