html-proofer: bump nokogiri to 1.16.0 to fix build

Builds fail on MacOS, with "incompatible function pointer types" due to
an outdated version of nokogiri.

Therefore, we pin nokogiri to 1.16.0.

This follows the same fix that was applied and merged in:
https://github.com/NixOS/nixpkgs/pull/285822

pkgs/tools/misc/html-proofer/Gemfile.lock

@@ -22,7 +22,7 @@ GEM
     html-proofer (5.0.8)
       addressable (~> 2.3)
       async (~> 2.1)
-      nokogiri (~> 1.13)
+      nokogiri (~> 1.16.0)
       pdf-reader (~> 2.11)
       rainbow (~> 3.0)
       typhoeus (~> 1.3)
@@ -30,7 +30,7 @@ GEM
       zeitwerk (~> 2.5)
     io-event (1.2.3)
     mini_portile2 (2.8.4)
-    nokogiri (1.15.4)
+    nokogiri (1.16.0)
       mini_portile2 (~> 2.8.2)
       racc (~> 1.4)
     pdf-reader (2.11.0)

pkgs/tools/misc/html-proofer/gemset.nix

@@ -140,10 +140,10 @@
     platforms = [];
     source = {
       remotes = ["https://rubygems.org"];
-      sha256 = "0k9w2z0953mnjrsji74cshqqp08q7m1r6zhadw1w0g34xzjh3a74";
+      sha256 = "1l8b0i24h4irivyhwy9xmkjbggw86cxkzkiqdqg0jpcp9qc8h4rl";
       type = "gem";
     };
-    version = "1.15.4";
+    version = "1.16.0";
   };
   pdf-reader = {
     dependencies = ["Ascii85" "afm" "hashery" "ruby-rc4" "ttfunk"];