Add webrick to Gemfile for Ruby 3.x support

Jekyll uses webrick in development mode, and expects it to ship with Ruby. But Ruby 3.0 stopped bundling webrick, which causes an error when you run `jekyll serve` using ruby 3.x.
Jekyll 4.3.0—released Oct 2022—adds webrick as a dependency, but GitHub Pages still uses Jekyll 3.9.2 and [has no immediate plans](github/pages-gem#651 (comment)) to upgrade to Jekyll 4. Adding webrick to our Gemfile (per [this thread](jekyll/jekyll#8523)) solves this problem in the meantime.

Gemfile

@@ -1,3 +1,4 @@
 source "https://rubygems.org"
 
 gem "github-pages", group: :jekyll_plugins
+gem "webrick", "~> 1.7"

Gemfile.lock

@@ -26,10 +26,10 @@ GEM
     eventmachine (1.2.7)
     eventmachine (1.2.7-x64-mingw32)
     execjs (2.8.1)
-    faraday (2.6.0)
+    faraday (2.7.2)
       faraday-net_http (>= 2.0, < 3.1)
       ruby2_keywords (>= 0.0.4)
-    faraday-net_http (3.0.1)
+    faraday-net_http (3.0.2)
     ffi (1.15.5)
     ffi (1.15.5-x64-mingw32)
     forwardable-extended (2.6.0)
@@ -204,28 +204,26 @@ GEM
     kramdown-parser-gfm (1.1.0)
       kramdown (~> 2.0)
     liquid (4.0.3)
-    listen (3.7.1)
+    listen (3.8.0)
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)
     mercenary (0.3.6)
-    mini_portile2 (2.8.0)
+    mini_portile2 (2.8.1)
     minima (2.5.1)
       jekyll (>= 3.5, < 5.0)
       jekyll-feed (~> 0.9)
       jekyll-seo-tag (~> 2.1)
-    minitest (5.16.3)
-    nokogiri (1.13.9)
+    minitest (5.17.0)
+    nokogiri (1.14.0)
       mini_portile2 (~> 2.8.0)
       racc (~> 1.4)
-    nokogiri (1.13.9-x64-mingw32)
-      racc (~> 1.4)
     octokit (4.25.1)
       faraday (>= 1, < 3)
       sawyer (~> 0.9)
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
     public_suffix (4.0.7)
-    racc (1.6.0)
+    racc (1.6.2)
     rb-fsevent (0.11.2)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
@@ -254,8 +252,8 @@ GEM
     unf (0.1.4)
       unf_ext
     unf_ext (0.0.8.2)
-    unf_ext (0.0.8.2-x64-mingw32)
     unicode-display_width (1.8.0)
+    webrick (1.7.0)
     zeitwerk (2.6.6)
 
 PLATFORMS
@@ -264,6 +262,7 @@ PLATFORMS
 
 DEPENDENCIES
   github-pages
+  webrick (~> 1.7)
 
 BUNDLED WITH
-   2.1.4
+   2.4.3