build: Use real Jekyll (not GitHub pages)

Since I'm building it with an Action now, I don't need to depend on
GitHub pages' locked down build process. That prevented _plugins from
running. Moving away from the old gem also simplifies _config.yml.

Note that for GitHub metadata we need the fix from
github/pages-gem#399.

Gemfile

@@ -1,5 +1,24 @@
 source 'https://rubygems.org'
-gem 'github-pages', group: :jekyll_plugins
+gem 'jekyll', '4.2.0'
+
+group :jekyll_plugins do
+  gem "jekyll-feed"
+  gem "jekyll-mentions"
+  gem "jekyll-redirect-from"
+  gem "jekyll-sitemap"
+  gem "jemoji"
+
+  # github pages
+  gem "jekyll-coffeescript"
+  gem "jekyll-default-layout"
+  gem "jekyll-gist"
+  gem "jekyll-github-metadata"
+  gem "jekyll-optional-front-matter"
+  gem "jekyll-paginate"
+  gem "jekyll-readme-index"
+  gem "jekyll-relative-links"
+  gem "jekyll-titles-from-headings"
+end
 
 # ruby 3.0.0 doesn't bundle this, and jekyll as of 3.9.0, the version Github Pages
 # uses, doesn't have the fix yet

_config.yml

@@ -14,6 +14,7 @@ kramdown:
 future: false
 jailed: false
 gfm_quirks: paragraph_end
+github: [metadata]
 
 exclude:
   - Gemfile
@@ -26,24 +27,6 @@ exclude:
   - tags-list
   - vendor/
 
-plugins:
-  - jekyll-feed
-  - jekyll-mentions
-  - jekyll-redirect-from
-  - jekyll-sitemap
-  - jemoji
-
-  # gh pages
-  # - jekyll-coffeescript
-  # - jekyll-default-layout
-  # - jekyll-gist
-  # - jekyll-github-metadata
-  # - jekyll-optional-front-matter
-  # - jekyll-paginate
-  # - jekyll-readme-index
-  # - jekyll-relative-links
-  # - jekyll-titles-from-headings
-
 defaults:
   -
     scope: