Update all dependencies to their latest (still on Ruby 2.x)

This appears to include some small changes to syntax highlighting
within f-strings, though nothing which appears incorrect.

Gemfile.lock

@@ -1,19 +1,19 @@
 GEM
   remote: https://rubygems.org/
   specs:
-    addressable (2.8.1)
-      public_suffix (>= 2.0.2, < 6.0)
+    addressable (2.8.7)
+      public_suffix (>= 2.0.2, < 7.0)
     colorator (1.1.0)
-    concurrent-ruby (1.1.8)
-    em-websocket (0.5.2)
+    concurrent-ruby (1.3.4)
+    em-websocket (0.5.3)
       eventmachine (>= 0.12.9)
-      http_parser.rb (~> 0.6.0)
-    ethon (0.15.0)
+      http_parser.rb (~> 0)
+    ethon (0.16.0)
       ffi (>= 1.15.0)
     eventmachine (1.2.7)
     eventmachine (1.2.7-x64-mingw32)
-    ffi (1.15.5)
-    ffi (1.15.5-x64-mingw32)
+    ffi (1.17.0)
+    ffi (1.17.0-x64-mingw32)
     forwardable-extended (2.6.0)
     html-proofer (4.4.3)
       addressable (~> 2.3)
@@ -24,7 +24,7 @@ GEM
       typhoeus (~> 1.3)
       yell (~> 2.0)
       zeitwerk (~> 2.5)
-    http_parser.rb (0.6.0)
+    http_parser.rb (0.8.0)
     i18n (0.9.5)
       concurrent-ruby (~> 1.0)
     jekyll (3.9.0)
@@ -46,37 +46,36 @@ GEM
       sass (~> 3.4)
     jekyll-watch (2.2.1)
       listen (~> 3.0)
-    kramdown (2.3.1)
+    kramdown (2.4.0)
       rexml
     kramdown-parser-gfm (1.1.0)
       kramdown (~> 2.0)
-    liquid (4.0.3)
-    listen (3.7.1)
+    liquid (4.0.4)
+    listen (3.9.0)
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)
     mercenary (0.3.6)
-    mini_portile2 (2.8.5)
-    neat (3.0.0)
-      sass (~> 3.4)
+    mini_portile2 (2.8.7)
+    neat (4.0.0)
       thor (~> 0.19)
     nokogiri (1.15.6)
       mini_portile2 (~> 2.8.2)
       racc (~> 1.4)
     nokogiri (1.15.6-x64-mingw32)
       racc (~> 1.4)
-    parallel (1.22.1)
+    parallel (1.26.3)
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
-    public_suffix (5.0.0)
-    racc (1.7.3)
+    public_suffix (5.1.1)
+    racc (1.8.1)
     rainbow (3.1.1)
-    rake (12.3.3)
-    rb-fsevent (0.11.0)
-    rb-inotify (0.10.1)
+    rake (13.2.1)
+    rb-fsevent (0.11.2)
+    rb-inotify (0.11.1)
       ffi (~> 1.0)
-    rexml (3.3.3)
+    rexml (3.3.5)
       strscan
-    rouge (3.26.0)
+    rouge (3.30.0)
     safe_yaml (1.0.5)
     sass (3.7.4)
       sass-listen (~> 4.0.0)
@@ -85,12 +84,12 @@ GEM
       rb-inotify (~> 0.9, >= 0.9.7)
     strscan (3.1.0)
     thor (0.20.3)
-    typhoeus (1.4.0)
+    typhoeus (1.4.1)
       ethon (>= 0.9.0)
-    wdm (0.1.1)
-    webrick (1.7.0)
+    wdm (0.2.0)
+    webrick (1.8.1)
     yell (2.2.2)
-    zeitwerk (2.6.1)
+    zeitwerk (2.6.17)
 
 PLATFORMS
   ruby