deps: Add logger gem for Ruby 4.0+

- Ruby 4.0 removed `logger` from the default gems, but Jekyll 4.4
  still requires it at load time, causing `bundle exec jekyll build`
  to fail with `cannot load such file -- logger`.
- Declare `logger` explicitly so the build works on Ruby 4.0+ without
  affecting older Rubies that still ship it.

Gemfile

@@ -5,3 +5,4 @@ gem "jekyll-feed"
 gem "jekyll-redirect-from"
 gem "webrick"
 gem "rouge"
+gem "logger"

Gemfile.lock

@@ -91,6 +91,7 @@ GEM
     listen (3.9.0)
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)
+    logger (1.7.0)
     mercenary (0.4.0)
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
@@ -172,6 +173,7 @@ DEPENDENCIES
   jekyll (~> 4.4)
   jekyll-feed
   jekyll-redirect-from
+  logger
   rouge
   webrick