Bundler could not find compatible versions for gem 'jekyll-feed'

Bug

While trying to install gem github-pages needed to deploy this site, I ran into dependency conflict

Bundler found conflicting requirements for the Ruby version:
  In Gemfile:
    github-pages was resolved to 1, which depends on
      Ruby (~> 1.9.3)

    jekyll-feed (~> 0.12) was resolved to 0.16.0, which depends on
      Ruby (>= 2.5.0)

  Current Ruby version:
    Ruby (= 3.0.0)

Bundler could not find compatible versions for gem "jekyll-feed":
  In snapshot (Gemfile.lock):
    jekyll-feed (= 0.16.0)

  In Gemfile:
    github-pages was resolved to 36, which depends on
      jekyll-feed (= 0.2.3)

    minima (~> 2.5) was resolved to 2.5.1, which depends on
      jekyll-feed (~> 0.9)

Deleting your Gemfile.lock file and running `bundle install` will rebuild your snapshot from scratch, using only
the gems in your Gemfile, which may resolve the conflict.

Solution

Comment out this line in Gemfile:

# gem "jekyll", "~> 3.9.2"

Gem github-pages will import older version of jekyll and everything should work normally.

Apparently, gem github-pages relies on old version of jekyll, and imports it. I don’t know why is it done this way. It’s mentioned in Gemfile comments, though. But I rarely read comments in Gemfile, because usually there is usually only broilerplate there.