Rename Nokogiri::HTML module to Nokogiri::HTML4

Starting version 1.20.0 the module name changed and the old
name is aliased to preserve backwards-compatibility.
This might be deprecated in future versions, so better
change it right away.

See https://github.com/sparklemotion/nokogiri/blob/main/CHANGELOG.md#1120--2021-08-02

src/api/test/functional/webui/spider_test.rb

@@ -111,7 +111,7 @@ def crawl
       end
       body = nil
       begin
-        body = Nokogiri::HTML::Document.parse(page.source).root
+        body = Nokogiri::HTML4::Document.parse(page.source).root
       rescue Nokogiri::XML::SyntaxError
         # puts "HARDCORE!! #{theone}"
       end