fix: Gemfile & Gemfile.lock to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-6228056

Gemfile

@@ -1,7 +1,7 @@
 source "https://rubygems.org"
 
 gem "jekyll", "~> 3.9", ">= 3.9.0"
-gem "html-proofer", "~> 3.19", ">= 3.19.2"
+gem "html-proofer", "~> 4.0", ">= 4.0.0"
 gem "jekyll-remote-theme", ">= 0.4.3"
 gem "jekyll-open-sdg-plugins", "0.0.16"
 gem "jekyll-get-json", "~> 0.0.2"

Gemfile.lock

@@ -1,7 +1,7 @@
 GEM
   remote: https://rubygems.org/
   specs:
-    addressable (2.8.1)
+    addressable (2.8.6)
       public_suffix (>= 2.0.2, < 6.0)
     colorator (1.1.0)
     concurrent-ruby (1.1.10)
@@ -12,16 +12,17 @@ GEM
     ethon (0.16.0)
       ffi (>= 1.15.0)
     eventmachine (1.2.7)
-    ffi (1.15.5)
+    ffi (1.16.3)
     forwardable-extended (2.6.0)
-    html-proofer (3.19.4)
+    html-proofer (4.4.3)
       addressable (~> 2.3)
       mercenary (~> 0.3)
       nokogiri (~> 1.13)
       parallel (~> 1.10)
       rainbow (~> 3.0)
       typhoeus (~> 1.3)
       yell (~> 2.0)
+      zeitwerk (~> 2.5)
     http_parser.rb (0.8.0)
     i18n (0.9.5)
       concurrent-ruby (~> 1.0)
@@ -63,18 +64,18 @@ GEM
       rb-inotify (~> 0.9, >= 0.9.10)
     mercenary (0.3.6)
     mini_portile2 (2.8.5)
-    nokogiri (1.15.4)
+    nokogiri (1.15.6)
       mini_portile2 (~> 2.8.2)
       racc (~> 1.4)
-    nokogiri (1.15.4-x86_64-darwin)
+    nokogiri (1.15.6-x86_64-darwin)
       racc (~> 1.4)
-    nokogiri (1.15.4-x86_64-linux)
+    nokogiri (1.15.6-x86_64-linux)
       racc (~> 1.4)
-    parallel (1.22.1)
+    parallel (1.24.0)
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
-    public_suffix (5.0.1)
-    racc (1.7.1)
+    public_suffix (5.0.5)
+    racc (1.7.3)
     rainbow (3.1.1)
     rb-fsevent (0.11.2)
     rb-inotify (0.10.1)
@@ -88,17 +89,18 @@ GEM
     sass-listen (4.0.0)
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
-    typhoeus (1.4.0)
+    typhoeus (1.4.1)
       ethon (>= 0.9.0)
     yell (2.2.2)
+    zeitwerk (2.6.13)
 
 PLATFORMS
   ruby
   x86_64-darwin-21
   x86_64-linux
 
 DEPENDENCIES
-  html-proofer (~> 3.19, >= 3.19.2)
+  html-proofer (~> 4.0, >= 4.0.0)
   jekyll (~> 3.9, >= 3.9.0)
   jekyll-get-json (~> 0.0.2)
   jekyll-open-sdg-plugins (= 0.0.16)