I stumbled upon this question when I ran into the issue of trying to set up github pages. It seems that in the latest version of ruby that is installed with homebrew webrick is not included by default. Here is the github issue: https://github.com/github/pages-gem/issues/752. Recommendations are to either use ruby 2.7 or to manually add webrick in using the command below:
bundle add webrick
and then repeating the command
bundle exec jekyll serve