Switch to Jekyll 4.2; bundle update

It is necessary to skip to 4.2, because 3.9 (in Debian) does not
support Ruby 3.0.
Also, remove GitHub Pages stuff and add some gems to make it all work.

Gemfile

@@ -8,17 +8,19 @@ source "https://rubygems.org"
 #
 # This will help ensure the proper Jekyll version is running.
 # Happy Jekylling!
-gem "jekyll", "~> 3.8.3"
+gem "jekyll", "~> 4.2.0"
 
 # Use this theme instead.
-gem "jekyll-theme-hydeout", "~> 3.4"
+gem "jekyll-theme-hydeout", "~> 4.2.0"
-
-# If you want to use GitHub Pages, remove the "gem "jekyll"" above and
-# uncomment the line below. To upgrade, run `bundle update github-pages`.
-# gem "github-pages", group: :jekyll_plugins
 
 # If you have any plugins, put them here!
 group :jekyll_plugins do
   gem "jekyll-admin"
   gem "jekyll-feed", "~> 0.6"
 end
+
+# Necessary for jekyll-admin to work (Webrick is missing in Ruby >= 3.0).
+gem "webrick"
+
+# Support GFM dialect of Markdown.
+gem "kramdown-parser-gfm"

Gemfile.lock

@@ -10,7 +10,7 @@ GEM
       eventmachine (>= 0.12.9)
       http_parser.rb (~> 0)
     eventmachine (1.2.7)
-    faraday (1.9.3)
+    faraday (1.10.0)
       faraday-em_http (~> 1.0)
       faraday-em_synchrony (~> 1.0)
       faraday-excon (~> 1.1)
@@ -36,45 +36,50 @@ GEM
     ffi (1.15.5)
     forwardable-extended (2.6.0)
     http_parser.rb (0.8.0)
-    i18n (0.9.5)
+    i18n (1.10.0)
       concurrent-ruby (~> 1.0)
-    jekyll (3.8.7)
+    jekyll (4.2.2)
       addressable (~> 2.4)
       colorator (~> 1.0)
       em-websocket (~> 0.5)
-      i18n (~> 0.7)
+      i18n (~> 1.0)
-      jekyll-sass-converter (~> 1.0)
+      jekyll-sass-converter (~> 2.0)
       jekyll-watch (~> 2.0)
-      kramdown (~> 1.14)
+      kramdown (~> 2.3)
+      kramdown-parser-gfm (~> 1.0)
       liquid (~> 4.0)
-      mercenary (~> 0.3.3)
+      mercenary (~> 0.4.0)
       pathutil (~> 0.9)
-      rouge (>= 1.7, < 4)
+      rouge (~> 3.0)
       safe_yaml (~> 1.0)
+      terminal-table (~> 2.0)
     jekyll-admin (0.11.0)
       jekyll (>= 3.7, < 5.0)
       sinatra (~> 1.4)
       sinatra-contrib (~> 1.4)
-    jekyll-feed (0.16.0)
+    jekyll-feed (0.15.1)
       jekyll (>= 3.7, < 5.0)
     jekyll-gist (1.5.0)
       octokit (~> 4.2)
     jekyll-paginate (1.1.0)
-    jekyll-sass-converter (1.5.2)
+    jekyll-sass-converter (2.2.0)
-      sass (~> 3.4)
+      sassc (> 2.0.1, < 3.0)
-    jekyll-theme-hydeout (3.7.1)
+    jekyll-theme-hydeout (4.2.0)
-      jekyll (~> 3.4)
+      jekyll (>= 3.6, < 5.0)
       jekyll-feed (~> 0.6)
       jekyll-gist (~> 1.4)
       jekyll-paginate (~> 1.1)
     jekyll-watch (2.2.1)
       listen (~> 3.0)
-    kramdown (1.17.0)
+    kramdown (2.3.1)
+      rexml
+    kramdown-parser-gfm (1.1.0)
+      kramdown (~> 2.0)
     liquid (4.0.3)
     listen (3.7.1)
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)
-    mercenary (0.3.6)
+    mercenary (0.4.0)
     multi_json (1.15.0)
     multipart-post (2.1.1)
     octokit (4.22.0)
@@ -88,17 +93,15 @@ GEM
       rack
     rack-test (1.1.0)
       rack (>= 1.0, < 3)
-    rb-fsevent (0.11.0)
+    rb-fsevent (0.11.1)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    rouge (3.27.0)
+    rexml (3.2.5)
+    rouge (3.28.0)
     ruby2_keywords (0.0.5)
     safe_yaml (1.0.5)
-    sass (3.7.4)
+    sassc (2.4.0)
-      sass-listen (~> 4.0.0)
+      ffi (~> 1.9)
-    sass-listen (4.0.0)
-      rb-fsevent (~> 0.9, >= 0.9.4)
-      rb-inotify (~> 0.9, >= 0.9.7)
     sawyer (0.8.2)
       addressable (>= 2.3.5)
       faraday (> 0.8, < 2.0)
@@ -113,16 +116,22 @@ GEM
       rack-test
       sinatra (~> 1.4.0)
       tilt (>= 1.3, < 3)
+    terminal-table (2.0.0)
+      unicode-display_width (~> 1.1, >= 1.1.1)
     tilt (2.0.10)
+    unicode-display_width (1.8.0)
+    webrick (1.7.0)
 
 PLATFORMS
-  ruby
+  x86_64-linux
 
 DEPENDENCIES
-  jekyll (~> 3.8.3)
+  jekyll (~> 4.2.0)
   jekyll-admin
   jekyll-feed (~> 0.6)
-  jekyll-theme-hydeout (~> 3.4)
+  jekyll-theme-hydeout (~> 4.2.0)
+  kramdown-parser-gfm
+  webrick
 
 BUNDLED WITH
-   2.1.4
+   2.3.5