Summary

Ruby 3 is starting to drop built-in RubyGems, like csv and webrick. Jekyll relies on these, so let's release a Jekyll 3 minor version that includes them.

Use 3.10-stable branch instead of master when filing PRs.

Tasks

• Add csv: Backport add-csv-dependency from #9522 to Jekyll 3 #9616 (dropping as of Ruby 3.4)
• Add webrick: 3.10-stable: Add webrick as a dependency #9620 (dropped in Ruby 3.0)