Upgrade nokogiri to fix vulnerabilities

Upgrading nokogiri to 1.16.5 fixes a vulnerability in the included
version of libxml2.

See:

- https://github.com/gma/nesta/security/dependabot/26
- https://gitlab.gnome.org/GNOME/libxml2/-/issues/720

Gemfile.lock

@@ -41,7 +41,7 @@ GEM
       rb-inotify (>= 0.9)
       rb-kqueue (>= 0.2)
     mini_mime (1.1.2)
-    mini_portile2 (2.8.5)
+    mini_portile2 (2.8.6)
     minitest (5.18.0)
     minitest-reporters (1.6.0)
       ansi
@@ -55,7 +55,7 @@ GEM
       unicorn (>= 4.5)
     mustermann (3.0.0)
       ruby2_keywords (~> 0.0.1)
-    nokogiri (1.16.2)
+    nokogiri (1.16.5)
       mini_portile2 (~> 2.8.2)
       racc (~> 1.4)
     public_suffix (5.0.1)