Switch to nokogiri 1.12, since it includes html5 support directly now

lib/theme_check/checks/valid_html_translation.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require 'nokogumbo'
+require 'nokogiri'
 
 module ThemeCheck
   class ValidHTMLTranslation < JsonCheck

lib/theme_check/html_visitor.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
-require "nokogumbo"
+require "nokogiri"
 require "forwardable"
 
 module ThemeCheck

theme-check.gemspec

@@ -25,5 +25,5 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
 
   spec.add_dependency('liquid', '>= 5.0.1')
-  spec.add_dependency('nokogumbo')
+  spec.add_dependency('nokogiri', '>= 1.12')
 end